I'm just gonna talk about Capcom here because it's their side that I have the most beef with.
I have lost faith in Capcom as a fighting game dev by this point. The last fighting game they made that interested me was Street Fighter X Tekken, and that was back in 2012, over a decade ago by now. Street Fighter 4 and 5 look horrible, and MvC Infinite was such a dud that I'm shocked Capcom is even still making Marvel games by this point, I would've thought that Infinite would've been the nail in the coffin for their rights to the Marvel license. And I have zero expectations for
SF 5 EDIT: Street Fighter 6. (my bad)
Now, with that said, as someone who spent a lot of time playing MvC 2 on my PS3 back in the day, and as someone who glanced at MvC 3's roaster, there are a number of roaster omissions throughout the MvC sub-series that I simply must rant about. Right now I'm going to list all of the Capcom characters off the top of my head that I've always wanted to see in a MvC game but have never made the cut for whatever reason. Here would be my Capcom choices for the MvC 4 roaster:
STREET FIGHTER:
-Urien. given the "super hero" theme, he and especially Gill would fit right in, since he's basically an ancient Greek fighter in super villain form who wields super powers such as energy shields and throwable lighting orbs.
-Gill. I can't believe Capcom hasn't put Gill in a MvC game yet. He's even more super-human that Urien. Gill was basically a manga-take on an ancient Greek god. His supers would work perfectly as assists. Put him in MvC 4, Capcom!
-Twelve. Necro is a fine choice for the MvC 4 roaster, but twelve is an even better choice since he's basically just a souped-up version of Necro, with more superhuman abilities like gliding and morphing. On top of that, his supers would work perfectly as assists. And on top of THAT, his concept/design kinda reminds me of the final boss from MvC2 somewhat, especially the "mercury man" animations:
IDK I always thought he reminded me of Twelve a bit. On top of that, the MvC games are chocked full of Marvel henchmen like Sentinel and juggernaut, so why shouldn't an evil foot soldier like Twelve be at home in this series? Put him in, Capcom.
-Q. Again, given the "super hero" theme of the MvC games, Q would be a good choice for the roaster since he was basically a boss character in Third Strike and possessed super-human stamina and abilities. He would make a great villain addition for the Capcom side.
MEGA MAN:
-Yellow Devil. Considering that there have been tons of larger-than-life heroes and villains on the Marvel side of the roaster for these games like Sentinel, Juggernaut, and Hulk, I would LOVE to see Yellow Devil added to MvC 4's roaster, especially since Yellow Devil is arguably the single most iconic henchmen in the entire Mega Man series. I love Yellow Devil. He's always been the freakiest Mega Man baddie in my mind, and I want to see him put into a MvC game so badly.
-Protoman. Has Protoman ever been included in a Capcom Vs game? I don't believe he has, which is why I want to see him added to the roaster for MvC 4. I know Zero has, but I don't believe Protoman has. Protoman would fit right into the Marvel Vs series considering the Mega Man games are basically a Tokusatsu super hero series similar to other superhero manga like Astro Boy. He definitely has the comic book vibe and cool factor for a Marvel Vs game.
RESIDENT EVIL:
-Nemesis. I can't believe Nemesis hasn't been in a MvC games yet. If other Resident Evil characters like Jill can make it into the game, then shouldn't Nemesis?? He's the most iconic villain of the Resident Evil series, and his monstrous appearance & stature along with his supernatural strength combined with the other hellish powers he possesses would make him right at home with all of the bat-shit scary Marvel super monsters like Blackheart and Shuma Gorath.
That's all I can recommend for now. I'll probably add more Capcom characters to my roaster wish-list soon, along with maybe some Marvel characters as well.