Europa Universalis IV is a good game with a lot of minor issues, and the goal of Fun and Balance is to fix such small issues, to provide a "better vanilla" experience.
I created the mod long time ago, and I've been updating it every major patch. As the game changes, so does the mod, and it's been a while since I last documented the full list of what the mod is doing exactly.
I drop features from the mod a lot - usually problem areas are quite obvious to devs as much as to me, so they tend to get addressed eventually, and then my fixes aren't necessary. Occasionally a fix outlives its usefulness, but I've been trying to clean things up if they're not pulling their weight.
The mod doesn't try to affect game difficulty to a significant degree. It used to be somewhat harder than vanilla, but due to recent changes in favor systems, it's probably about even now.
So here's the full list of changes Fun and Balance makes, with their rationale. This is a fairly long list, with more important ones first.
More diplomatic relations
There are +2 base relationships, from 4 to 6. This is probably the most impactful change, and it used to be +4 before base game started adding a lot of ways to increase your relationships like Strong Duchies estate priviledge, and various monuments.
This generally increases difficulty of expanding. AI is very reluctant to go over its limit, so this change dramatically increases density of AI alliances, and makes expansion harder. Not just because there are now 2 more countries to help your target, denser network of alliances makes it a lot harder to fully isolate multiple targets diplomatically, so expansion and coalition management are both considerably harder.
As allies are a lot more important defensively than offensively, this change is a lot less useful for the player, but it enables having long term small vassals, royal marriages, or guarantees without obsessing over diplomatic slot utilization. A lot of options from the base game were very rarely used because of overly low limit.
In recent patch, with curry favors being arguably overpowered, this is less true. 2 extra allies are worth a lot more offensively than they used to. I think it still makes the game harder overall, but less so than it used to.
All strait bridges removed
Now you'll need transport ships to cross even narrow straits. This makes navies far more important, not something you need just once a game to invade Great Britain.
This surprisingly makest the game a bit harder, as AI is a lot better at dealing with such local transport than with long range invasions. Many scenarios like getting landlocked allies like Austria or Poland to help you fight Ottomans or Denmark, it's not so easy anymore.
I had to leave one, or the game would crash.
Eurocentric Institutions
Institutions were an interesting system, but they completely broke the balance between Europe and Rest of the Old World. They spread far too fast, so tech gap between Europe and India, China, or East Africa is near zero, or even goes the wrong way. That means the whole Old World plays pretty much the same way, and there's no threat of European invasion.
So the following changes have been introduced to slow down institution spread:
- passive spread of institutions ("neighboing province has X", "friendly neighboring province has X", and "X embraced") between neighboring provinces slowed down by half - but all other spread just like before
- spread of institutions only happens if previous institution is already present, notably you can't really get Colonialism without getting Renaissance first
- Colonialism can only start in Europe, you can still get it as non-European from your CNs
- dev pushing for institutions made 20% less effective - this isn't a huge change, but it prevents Korea or Ming from randomly dev pushing all institions. Realistically dev pushing as soon as possible is still optimal strategy for Rest of the Old World.
- Harar Jugol tier 1 monument nerfed - it was giving East African every institution for free really early and breaking the system, tiers 2-3 work exactly like before
This delays spread of early/mid game institutions and should increase mid-tier tech gap somewhat, from 1-2 to 3-4. Late game none of the institutions are region-locked, so the tech gap will go away eventually, just more slowly.
The mod also reduces ahead of tech penalty from 10%/year to 1%/year. But notably you'll still be paying penalty for not having relevant institution, even if that institution didn't spawn, so realistically you can get techs 1-2 tech ahead of time.
Between these two changes, Europeans can be scary again. But I've also seen AI Delhi exceed AI Commonwealth's tech levels even with institution penalties, so don't expect return to EU3 style tech gap here.
Religious leagues for all Christian denominations
In vanilla Catholic vs Protestant religious leagues can trigger. The mod adjusts the code so that all Christan denominations can do it. If 4/7 electors follow specific non-Catholic denomination, they can even happen very early, long before the usual time.
In a typical game, Protestant is still the most likely to trigger, with smaller chance of a Reformed league, and very remote chance of a Hussite one. But if you're playig with a custom non-Catholic nations, or specifically trying to spread some other religion like Hussite, Orthodox, Coptic, or Anglican, you can now do it.
More formable countries
Every culture has primary tag, and I made it possible to form most of them, changing your traditions, and missions. Some are not included if there are already obvious regional goals like Bharat, China, Russia etc. AI will also occasionally do this.
There aren't any obviously overpowered choices here, as end game tag formables usually get a lot better traditons and missions than regular countries. You can get a few decet ones, but it's mostly for more roleplaying options.
The "end game tag" check for formable tags is also disabled for the player, but AI playing as end game tag will still not form any other tags.
Disabled anti-player mechanics
EU4 is mostly fair between player and AI, but it has a few baffling mechanics where AI gets to cheat. These are removed.
Naval attrition form being at see, player-only in vanilla, is removed.
Call for Peace, also affecting only the player and never the AI, has no consequences now.
All coutries with capital in Asia can get Mandate of Heaven
This mostly gives additional options to Muslims and Hindus. Or you could go crazy, move Pope's capital to Jerusalem, and make the Pope be Emperor of China as well.
Restore Roman Empire works with subjects
Restore Roman Empire has extremely long list of provinces it needs, and they needed to be held directly, so a random PU could block you from getting it. Changed the decision so you can use it if (non tributary) subject holds them.
Trade map changes
Vanilla trade map makes it impossible for Asian to benefit from New World trade, just to let Europeans get Philippines. While historically it makes some small sense, it almost never happens in real gameplay - Europeans who colonize Asia get trade through Cape or Alexandria not through Mexico. Reversing direction of Pacific trade lets Asians get trade from New World's West Coast.
Casus Belli changes
CBs with locked out peace terms have been fixed to allow them. They're mostly useless or extremely niche, in base game. You'll still pay full cost for any such term, same you would without a CB. The biggest effect on the game is that you can declare war on your rival in December 1444 without waiting for fabricated claims and get some (undiscounted) land.
If both you and your subject have religious ideas, you can get CB on a country that your subject borders, but you don't. This doesn't significantly increase your ability to get CBs, as you could usually get one province to border your target, but it reduces bordergore slightly.
Coalition wargoal is defend capital (like it was in early patches), not win battles. Winning battles wargoal in EU4 has a lot of issues, and normally this is fine, but for coalition wars, you can get to situations where you're winning in every objective way, didn't lose a single battle, but your allies accumulated so many tiny battles lost that it would take you decades to convince the coalition to end war, while they can stab hit you every month. This isn't the most common, but it happened enough times that I decided to change it.
As defender in war you get 50% AE discount, instead of vanilla's 25%. EU4 makes being defender awful, as you don't get any CB discounts, so this brings some balance into it. It's still better to be the attacker generally, as you get your choice of CB, and most give decent discounts.
Maximum amount of gold you can get from a peace deal is doubled from 25% of warscore to 50%. This makes it slightly more worthwhile to get something from war when you don't want to take land. It's still far less than what EU3 or early EU4 allowed.
AI insistence on staying in war due to war being too short reduced by 1/3.
Power projection changes
I'd love to replace rival system completely, so that any Great Power can rival any other Great Power within range. Usually you get very limited choice of rivals beyond very early game. Sadly this is not possible, the rival system is nearly hardcoded.
The mod increases max distance for rival by 1/3, which seems to be the right number to give some flexibility without too many weird rivalries.
Power projection from rival related issues increased somewhat, and decaying more slowly, mostly to avoid nonsensical situations where you're too strong to get any power projection, as game won't let you pick rivals. I'd much rather fix rival selection, but that's not possible to mod.
Subject and opinion changes
Max hard cap of dev for diplovassalizing increased from 100 to 300. You'll still need a lot of factors to get anywhere close to that number.
Opinion penalty for annexing vassals capped at -100. In base game it could build up to ridiculous numbers if one of vassals was impossible to annex for some reason while others kept getting annexed over longer time. It was always possible to reset it to 0 by not annexing vassals for 20 years, but tracking exact date is just awkward as game does not show it anywhere. Capping it improves gameplay.
Supporting independence gives bigger relation boost, can be done from lower starting opinion, and AI doesn't care as much about having too many relations when it can support independence. All this modestly increases independence support.
Colonial Nations now consider their own power (as well as that of their supporters) for liberty desire, but to balance it out gets -25% base liberty desire. In base game supporting independence of a colonial nation does not increase liberty desire at all. This overall makes holding onto colonial nations as a very weak colonizer harder, most commonly Portugal might struggle with it, if it loses its army in war and its rivals support CN's independence. But it's still a modest factor.
Quite a few vastly excessive opinion penalties got reduced to sensible levels.
Unlawful HRE territory is capped at -50 to prevent spam. Violating treaty of Tordesillas is capped at -50.
Pirated us cap changed from -100 to -25. It really made no sense that countries would be more bothered by some minor piracy than by declaring war on them and occupying all their land, and it interfered with diplomacy as AI just loves spamming piracy. Converted our culture penalty cut from -30 to -5 and capped at -100.
More building slots
Building limit increased by +1 base, and from +1 per 10dev to +1 per 5dev.
This was much more aggressive previously, but I toned it down once game made Courthouses not take building slots, effectively giving every province +1 slot.
Extra slots mostly allow a bit more flexibility with buildings, and they benefit smaller richer countries more. Especially AI as it's bad at using its slots well. Big poor countries which can't fill their slots benefit from it the least.
Unlock regional features
The mod lets everyone claim whole areas, not just individual provinces. It was locked to Russia in base game. I originally wanted to make it available at empire tier only, but last time I checked that didn't work without making separate empire versions of every government reform. In any case, this mostly reduces border gore, and saves you a modest amount of diplomatic power early game.
Catholic Holy orders are unlocked so all Catholic can use them, not just Iberians.
Covert actions changes
Covert actions are massively underpowered, so they're buffed across the board.
Stealing maps cost reduced 50 to 20. Infiltrate administration cost 40 to 20. Agitate for liberty cost 90 to 50. Slander merchants cost from 70 to 20.
Agitate for liberty and infiltrate administration unlocked without requiring any tech.
Supporting rebels made significantly cheaper and more effective.
Even with all this aggressive buffs, none of these are particularly strong.
Custom nation tweaks
Max tier of ideas increased from 4 to 10, at costs following the curve, so you'll spend a lot of points on high tiers.
Penalty for max ideas from same group removed, as it's mostly for roleplaying.
Max distance between provinces increased from 400 to 1000, again for roleplaying.
Base monarch skill changed from 2/2/2 to 3/3/3, which is actual average.
Religious rebels changes
Religious rebels increased in priority, so a province with missionary is likely to spawn religious rebels, even if other kinds are also possible. They also always want to convert your country, even Pagan rebels, and province targetting weights changed to be more interested in going after provinces they can convert or provinces with a missionary.
Religious Shift Decision
If your capital has a different religion, you can pay 3 stability to shift to that religion, similar to CK2. AI will not use this.
This is an alternative to the very frustrating experience of hoping that religious rebels will do what you want - but if you can use rebels, that's a way to get free conversions and not lose stability.
Balkan Changes
Europa Universalis IV makes a lot of highly questionable decisions for Balkans, trying to railroad Ottomans as end game boss, at expense of making a lot of things nonsensical. Ottomans do not need any help, they'll be end game boss even if we fix things.
Ahistorical "Byzantium" renamed to "East Rome". It is also primary tag for Greek culture. Concept of "Greece" as a country is an invention from 1800s', Greek-speaking Romans saw themselves as Romans well into EU4's end game. It's also possible to form it as Romania, Ottomans, or HRE, if you're Greek Orthodox.
Turkish moved to its own culture group.
"Carpathian" culture group removed. Slovak moved to West Slavic, as it was really nonsensical that it was a different culture group from Czech. The rest merged with South Slavic into "Balkan" culture group. Notably Greek is still not in it, and I'm wondering if it should (together with two micro-cultures in its group).
Ottoman decision to move capital to Constantinople does not do free culture and religion conversion.
You can declare wars in regency
This was a pointless restriction nobody needs.
Improved bad idea groups
Some idea groups - notably Naval and Maritime - are just awful, so I tried to improve them somewhat.
The mod used to give Naval extra leader slot so it can recruit some admirals, but game changed its mind and increased leader slots to such high numbers you'll never need more, especially not admirals.
It still gives Maritime +1 merchant and +50% light ship trade power. To be honest, it's still so low tier I'm not sure anyone would take it even with the buff.
War Exhaustion changes
In vanilla War Exhaustion is largely just a tax on diplomatic power. The mod changes it so cost to reduce it is doubled if you're at war. Originally I disabled it completely, but sometimes you get stuck at long term fake war due to your allies, and I didn't want you to be stuck with big war exhaustion in such case.
War Exhastion gets reduced at peace twice as fast. It also convinces AI to peace out twice as much.
Other changes
Tradition from battles doubled, so it's influenced somewhat more by actually fighting or not, and somewhat less by forts and idea choices.
Costs to change culture, move capital, or move main trade port halved, as they were all seriously excessive. In a typical game you'll not use any of these features.
Limit of idea groups from same category removed, but that's already an option when you start a new campaign.
You can customize it
If you want to customize your experience, I tried to keep different changes separate, so you can adjust or delete any tweaks.
Overall the mod isn't trying to make it a completely different game, just a better version of what it could be.
I play about one campaign per major patch, plus run a lot of AI simulations to see if the changes work as intended. But it's definitely possible that some of the changes have serious side effects. If you play Fun and Balance, and have issues with either fun or balance, let me know.