If Ultimate Alliance 3 is any indication, then they probably would allow it, yes they would. I think X-Men would be fine with Wolverine, Storm, Magneto, and Deadpool. MAYBE Psylocke, Cyclops, and Juggernaut to appease the Marvel 2 fans. Finally adding the Fantastic Four is a must, and I think they could work as a team-based character like the Ginyu Force in DragonBall FighterZ. As for Spider-Man, it feels like a streatch, but it would be nice to get a Marvel vs. Capcom version of the Sinister Six. Ideally Dr. Octopus, Green Goblin, Venom, Electro, Rhino, and Mysterio.
I also want to see villains from other Marvel franchises. The Mandarin, The Leader or Abomination, Red Skull or Baron Zemo, frickin Loki (I'm stunned that he wasn't in Infinite with all the MCU love. Loki is like, the MCU darling villain). The Avengers from the movies, as shilly as it sounds, provide a nice swath of Marvel characters to include. Guardians really don't need more than Rocket w/Groot. Maybe Star-Lord if there's room. Then there's other characters like Blade, Punisher, Daredevil, The Inhumans, The Eternals. SO MUCH POTENTIAL. But it's almost impossible to make one that covers all your bases and still leaves room for Capcom.
Maybe Capcom should pull an NRS and make a Marvel solo fighting game and a Capcom All-Stars fighting game and then maybe later return to MvC.