Besides rebalancing old characters and adding new ones, if we're treating this as "what if Capcom legitimately made a new update for the game," there would probably be some changes to aspects of the game besides those. So this page covers basically all the rest of that.
- Rollback netcode. (This is the highest-priority one by a longshot; if literally nothing else changed, this alone would still be incredible.)
- Better lobbies; ability to have instant rematches, set win limits, train while in the lobby. (If you added rollback netcode this would be the 2nd priority, so people actually have an easier time using it.)
- Ability to do up to best-of-5 ranked online matches (since that is the standard format for offline tournament play).
- Online training mode.
- Somewhere in the range of 4~8 more stages. Some good choices: Qliphoth, some DMC castle, Dead Rising Mall, Mega Man X Highway, something from Darkstalkers, something from Street Fighter; potential Marvel choices include Dr. Doom's Castle, Atlantis, and The Nexus of All Realities.
- The ability to play as Galactus vs. a friend who's using a normal team (offline-only). Maybe let them have Heroes & Heralds equipment.
- The ability to do offline versus battles using Heroes & Heralds equipment (seperate mode from standard versus).
- Gold-tinted version of Heroes & Heralds color for everyone.
- Survival mode for people who like that kind of thing. Maybe a basic menu with a few "items" you can buy for points like SFV's. Saves highscores.
- Up to 10 favorited teams saved.
- Upon first boot-up with no detected save data, a mandatory, unskippable basic tutorial narrated by Professor X (who appears in the background) with Ryu and 2 other characters versus some other 3 (Wolverine?) in the Danger Room begins. Similar to SFV's mandatory tutorial. Just goes over basic stuff like how to move, block, use normals, specials, supers, and swap teammates.
- Some basic text box tutorials that cover the super-basics of how 2D fighting games even work in general, then ones that address all the specific, named mechanics of UMvC3 (Advancing Guard, Team Hyper Combo, etc.), and then 1-5 paragraph-long ones for each character (including a section on what changed from Ultimate).
- Scrap every mission and replace them with practical combo ones. First a single normal into a single special. Then a grounded magic series into a different special. Then a grounded magic series into an air magic series. Then a really basic solo bnb that would be as close as possible to the previous one but with a super slapped on. Maybe some kind of explanation of what these are for somewhere. Include the ability to watch demonstrations (yes, UMvC3 is lacking that option).
- More modern training mode with things like "playback recorded action upon recovery from blockstun". Hitbox viewer as well.
- Ability to hit a button on the command list to get extremely basic tags on moves like "projectile", "overhead", etc. (basically what the little images listing properties say on the SRK wiki).
- Set some harmless non-competitive things to be unlockable via the points you earn from playing. Many casual players enjoy this kind of thing and it takes no effort and doesn't really hamper competitive play. Examples include sound tests, concept artwork, watchable movies/cutscenes, etc.
- Superhero "League" like SFV dojos. Maybe even a customizable stage for the League? Not sure how that one would work tbh.