I've got a good ol' 72-character dream roster for a MvC4 on hand, so two in particular that I'd like to see from each (not necessarily the ones that are most likely, just ones I'd like to see)...
Marvel:
Songbird - I'd really like some Thunderbolts acknowledgement, and her powers can basically let her fill any roll, since they're Green Lantern-esque, as she can create objects using solid sound.
Apocalypse - Like a legit Apocalypse, not that boss version from XvSF/MvSF. LIke a real fighter version. He's got so much potential, with his shape shifting and Celestial-tech armor. You could even have that boss version show up as a super.
Capcom
Bass.EXE - He's a bit of an edgey version of Bass, who was already sort of a edgey version of Mega Man, but he's got a ton of interesting things to pull from, since his moveset kept evolving throughout all six BN games. Give him a Gospel gun, the earth breaker, and some of his dark energy attacks, and a super that sets up the Life Shield, and we're golden.
Donovan - Donovan from Darkstalkers. I've also got Pyron on this list, who I'd think would also work really well. Heck any Darkstalkers character would, but Donny with his Stand Spirit Channeling powers and demon sword would just be really interesting.