If you take the new XCOM game, remove all strategic element, and just leave it as a sequence of battles, then what you've got is pretty much a roguelike:
- Random maps - not in XCOM right now, but the game would be a lot better if it had it
- Permadeath - already there
- Turn-based - already there
- Grid-based - already there
- Non-modal - what I want to streamline away
- Complexity - already there to an extent, can add more while streamlining campaign mode away
- Resource management - already there
- Hack'n'slash / player-vs-world - already there
- Exploration and discovery - sort of already there, training roulette will be big here as well
- Single player character - definitely not, you'll be leading a squad (but not the entire XCOM project)
- Monsters are similar to players - sort of true already (and definitely true with EXALT)
- Tactical challenge - definitely so
- ASCII display - I don't think that's really viable any more, most roguelikes moved beyond that already, some kind of simple and mostly symbolic 2D is modern equivalent of that
- Dungeons - sort of borderline true already if you use very broad definition of "dungeon"
- Numbers - already there, the few missing numbers (mobility, XP) need to be added, as some XCOM mods do
I'm not sure if there are any sensible cross-platform alternatives to going HTML5.
Minor random issues
- I feel it should be possible to get new soldiers to your squad as a reward, probably already somewhat leveled-up.
- I don't really care all that much about details of XCOM universe, their monster types etc. They're perfectly fine, but the game will probably end up remixing many other sources as well.
- Advanced weapons should generally come from capturing alien artifacts. Money, alien alloys, elerium, and weapon fragments all feel like unnecessary complexity.
- N-use-per-battle items would probably work better if they simply had cooldowns.
- Cover and overwatch mechanics are the key to everything and should be expanded upon.
- Visibility mechanic might need a bit of redesign, especially if it's for a relatively closed spaces of alien base/mothership.
- It would be great if it was possible to avoid all funny stuff related to triggering alien pods and just have aliens behave like your soldiers.
- I'd hate it if the best strategy turned out to be really slow turtling with overwatch and triggering one pod at a time, like in most vanilla XCOM: Enemy Unknown maps.
- The game should be designed to be fair but pretty much unwinnable. I'm sure someone will end up beating it anyway, but that should be a big deal.