Marvel side: Blackheart Omega Red Daredevil Juggernaut Venom Silver Samurai
Capcom side: Asura (yep, from the new Asura's Wrath) Anakiris Megaman John Talbian Jin Captain Commando
I'll just put 6 on each side, ok? Say what you will, but I think these would be the ones that bring something new and exciting to the table. Of course some of the moves would require a lot of work and creativity from Capcom. I mean, instead of Juggernaut being a huge hit box that punches and kicks people, or Cyclops being just another beam shooter (I know I didn't listed him).
With dedication they could do some awesome movesets with those characters.
Anyway, I know it's not other people's dream, but seeing Anakiris and Omega Red in the screen with this awesome comic style of UMVC3 would be enough for me.