For full list of changes read 1.6.1 (b) and 1.6.1 (a) release notes.
Philosophy of this mod is to move focus towards mechanics which lead to fun gameplay without requiring overly gamey tricks from the player, and to restore balance by helping the too weak, more than nerfing the too strong.
Other than taking away Muscovy's free settler (which isn't even a huge deal, since AI Muscovy takes exploration ideas first), there aren't really any outright nerfs included.
Helping BurgundyBurgundy starts in extremely perilous position between hostile HRE and extremely powerful France, and even with event that magically split it between France and the emperor (at 0 AE, free cores etc.) removed, it still rarely succeeds.
To give it better chance, the mod now moves Burgundy's capital to Franche-Comte, so it's a legal HRE member.
Historical situation is somewhat ambiguous here as feudal relations were complex, but making Burgundy HRE member as it vas in EU3 should greatly increases its chances of early survival against the Big Blue Blob without making it too powerful.
Helping RepublicsIntegrating a vassal no longer costs any republican tradition, only legitimacy. Tradition is extremely hard to get compared with legitimacy, penalties for it start as soon as you go below 100% (instead of only below 50%), and republics are really weak even with the mod reversing the most extreme nerfs, so there's no reason to punish them in another way by making significant vassal annexations impossible.
Helping AIAdvisor cost increase by year cut in half. It was extremely brutal, especially to non-Western tech countries, as incomes increase with tech level not with time, and not that fast even then. Even relatively sizeable non-player Western-tech countries had trouble paying for advisors, so only players with their vast and optimized empires were able to afford good advisors.
And now it's possible to have up to 10 active policies (instead of up to 5), and they no longer cost any monarch points. That's mostly here to help AI, as most policies are borderline worthless (-1 dip/month for -10% shipbuilding time etc.) and AI picks them basically at random, but it makes sense to reward different combinations of idea groups instead of making policies so expensive (in terms of most important resource - monarch points) that they'd almost never see any gameplay.
Deemphasize Unfun MechanicsThe mod uses 1.6.1's AE decay values (4/year), not 1.6.2's (2/year). It's easy to get very high AE levels now that vassalization generates huge amounts of AE, but at least decay was quite fast, so it balanced out somewhat.
With high AE from vassalizations with slow decay the game would end up with permanent global coalition against the player like it did in many previous patches, and that's not a fun place to be.
Emphasize ReligionNegative consequences of less than full religious unity increased from +2% revolt risk to +5% (+100% stability cost remains where it was), scaled proportionally. Conversion is usually (not for everyone) slightly easier in this mod due to missionary strength rebalance, and in recent patches there are many ways to increase religious unity even without conversion, so it's much easier to achieve high religious unity than it used to. In my experience religious unity is one of more fun mechanics, and its vanilla values are just too low, so it deserves a big boost.
And to give player more control over their destiny, more religious conversions are possible. Orthodox and Coptic can convert to and from all other Christian denominations. Eastern religions (Shinto, Confucianism, Buddhism) can convert to each other, and additionally Hinduism and Buddhism can convert to each other crossing religious group divide (so you can build a chain Sikhism-Hinduism-Buddhism-Shinto if you really want to).
Muslims can also now convert to each other, except the cost is -100 piety and -2 stability instead of -100 prestige, in line with already existing decisions to flip between Shia and Sunni (which has unnecessarily difficult preconditions).
Pretty much all the conversions enabled here happened multiple times historically, and they just give more gameplay options without breaking balance.
I tried to make sure that religion-specific country modifiers which ought to clear do clear, but it's very likely that I missed some of them. If you see a country modifier remaining which should be gone after conversion, just report it as a bug.
Emphasize Other Fun and Underused MechanicsMechanics which lead to interesting gameplay but don't see much action got emphasized.
War Exhaustion now reduce morale and defensiveness as it did before pre-1.6. In vanilla 1.6 War Exhaustion has no military significance whatsoever, it's basically just revolt risk increase, and that makes its offensive use basically pointless.
Culture conversion cost is lowered to 5/bt from previous values of 10/bt (which was still too high) and vanilla's completely insane 25/bt. I still don't expect it to be used much.
Improve relations mission gives 10 prestige and 25 diplo instead of just worthless 3 prestige. It might be a little much, but then insult rival mission is way easier and gives similar amounts, and a mission without meaningful rewards is just wasting mission slot - you should always have 3 meaningful missions to choose from.
Other Tweaks and FixesVassal annexation relationship penalty capped at -100, as it led to far too many ridiculous situations at -200, where vassals became pretty much permanently unannexable and had to be released from vassalage , annexed, and released as new vassals if you ever wanted to annex them.
Various bugs in tribal government reforms should now be fixed.
Everything is optionalI isolated each change as much as possible, so you can pick whichever parts of it you like and skip the rest. Pretty much the only part that's a bit harder to isolate is rebalancing missionary strength, but even that is not that hard.
The mod is not tested for compatibility with any other mod. It should work with UI-only modes, but it will probably require manual fixing for anything more complex.