Alright. Let's give this a shot. Spoiler tagged to save space.
[spoiler]I've already expressed my opinion on the Mega Man side of things. The Ultron/Sigma rumor going around is interesting. But I still feel X will be the only playable MM character. Definitely no other Mega Man type, definitely no other X series characters.
Street Fighter:
Chun Li is a given. Depending on space, Akuma might not make it. I'd prefer the MvC1 approach and let Ryu switch styles to Ken and Akuma. I think a character from Street Fighter V will be in, much like how Viper was put in MvC3 to promote SFIV. One of the yet to be revealed Season 2 characters could happen. Of the four we have right now, I'd say Rashid would fit the Versus style the best.
Darkstalkers:
I have no idea. There are many possible people to include, or they could just leave it with Morrigan. Hsien-Ko was a personal pick for Niitsuma, so I doubt she'd be in this game.
Resident Evil:
Likely much less RE5 representation. Dunno if they'd use anything from 6 or 7 or even Revelations. I don't really know enough about them. But I can almost guarantee Wesker will come back.
Final Fight:
Don't really have a reason for Haggar to not be included. Every other Final Fight character is either also a Street Fighter character or just too obscure. I'm sure Lucia would be awesome, but it just won't happen.
Devil May Cry:
Dante and Vergil are too popular to not include. If they would be the reboot versions or not, could go either way. Trish probably won't come back. Her use really went down in UMvC3.
As for other Capcom properties. The other characters in MvC3 are certainly possible. Likeliest to be cut I think would be Phoenix Wright, Viewtiful Joe, Amaterasu, and the Ghosts and Goblins characters, sadly. Dunno what they would pick out of the well of other properties. Maybe there will actually be a Monster Hunter representative this time?
Onto Marvel. Let's start with the Fox properties. A form of Wolverine will be there. Magneto will be there. Deadpool will be there. Doctor Doom will be there. Can't say there'd be too many else of them. Gambit has a solo movie coming up, right? Maybe he'd come back.
In the case of the Avengers, Hulk is a given. Thor is a given. Hawkeye is pretty likely. I could see Black Widow making it in, using a modification of Jill's MvC3 moveset. Black Panther is a safe bet. Vision is a good guess. Scarlet Witch is a good one. Falcon is likely. Doctor Strange is safe. Only one of the current roster (besides War Machine) I don't think will make it is Ant Man. I don't see his skill set translating well to a fighter. Same for Wasp, bu she at least can fly.
Villains: Thanos is super obvious. I think Loki will make it. Dormammu will be there. I don't think Shuma Gorath is safe. I don't think M.O.D.O.K. is safe. And there is still Ultron to think about.
Specifically concerning Guardians of the Galaxy, I'd say either Rocket comes back, or Groot makes it. Any more reps might be overdoing it. On a sorta related note, I don't think Nova will come back. Captain Marvel seems to play pretty similarly to how he did, so both would be redundant.
Anyone from the Netflix universe can make it. Although there might have been concerns before about how violent Punisher is. The rest are pretty damn likely.
Finally, Spider-Man. Now that that pesky Sony hubbub has passed, maybe we can get some of his rogues gallery this time? Preferably not Venom or Carnage, though. I believe Doctor Octopus was considered for MvC3. I'd pick him before anyone else.[/spoiler]