I don't really think the staff will go for a much different roster compared to MvC3 but well.
Anyone from Marvel side is okay to me, though if they take something based on the new Daredevil series (talking about Punisher) that would be real badass.
From Capcom side Rashid would be the perfect fit for a new Street Fighter rep., then C. Viper/Cap. Commando, and Victor/Lord Raptor/Donovan/B. B. Hood for Darkstalkers slot(s). Akuma would only be a choice if he is summoned by Anita like in MSH
By the way Rival Schools and Red Earth representants are clearly a must!
From Battle Circuit I don't mind anyone showing up besides the ostrich. I wish they would bring something related to Cannon Spike (that old Capcom shooter with assorted characters from various franchises, and also new faces). Seeing a Star Gladiator character in the current gen graphics would be outstanding. Maybe is too much to also hope for Son Son and Ruby Heart making a comeback?