I think Trucy Wright would make a great, creative addition with her magic tricks and Mr. Hat (plus it could mirror Phoenix Wright, with an Apollo-cameo special), but that's more far down the line / makes more sense as a late addition to MvC3. For Street Fighter, I always felt like Gill would make a good fit in the same way Viper did, and now I think the same for Rashid. I've always wanted Dr. Wily and Bass.EXE (way more than Megaman, anyway). "Arisen" and "Monster Hunter"/Felyne give you a huge pool of interesting moves and mechanics to incorporate into a character, especially if you throw in a weapon/class move. In MvC3 terms, bringing Regina would have been a cool way to bring back MvC2 Jill, but since there's a whole new slate, they could just have any RE character do that. Since this isn't MvC3, I hope they shake up Darkstalkers reps a little. People want Ryu from Breath of Fire, but Nina would be a cleaner pick and she's just as representative given she is in every single game in some form (plus magic characters are/were cool in MvC3). And Gene from God Hand is a "pay any amount of money for DLC" character for me.
Don't really care about the Marvel side anymore (especially if it is MCU focused). I guess someone like Machine Man would be cool, but more unlikely now than during the MvC3 era. I think characters like Black Bolt and Groot make more sense in a AntMan/Hawkeye position than full characters (for Medusa and Rocket Raccoon, respectively, who make sense to me as full blown characters).
Oh, one idea I had way back when fan-fiction about MvC3 rosters was viable was a Squirrel Girl special where Thanos would jump in with the Infinity Gauntlet, obliterate the opponent only for SG to then casually kick him so hard he gets Team Rocket'd and for the Infinity Gauntlet to fall in her hands which she immediately carelessly tosses it away. Guess that wouldn't really fit with the way MCU is heading lol. Plus Squirrel Girl is lame by now.