-Deoxys (though the lack of "true" tentacles could be a problem)
-Electabuzz (let's face it; Diddy
needs at least one good PSA)
-Black Knight (hint hint SDoom

)
-Darkrai (though Ganondorf already has a lot of "darkness" PSA's)
-Boba Fett (Snake needs more lasers)
-Blaziken (could be a C.Falcon clone though...)
-Frog (could his bubbles even be made to work though?)
-Ragna the Bloodedge (already a fighting game character, so thinking up the moveset would be easier)
-Nightmare (again, fighting game character)
-Sub-Zero (would be cool to see C.F use ice attacks. Also, another fighting game charatcer)
-Vergil (to go with the Dante PSA being made)
-MvC3 Zero (again, hint hint SDoom

)
I vote for either Etna or Psychryphias. Peach needs some epic PSAs!
Deoxys - Wouldn't be good without his tentacles moving properly
Electabuzz - The face looks so creepy ;~; but Diddy does need a good PSA
Black Knight - SDo0m is already doing this after Zero >.>
Darkrai - I have a planned moveset for this, and there's a working model port on Lucario being worked on.
Boba Fett - Hmm, wouldn't this consist too much of lasers?
Blaziken - Falcon with different animations and a different Side and Up B o:
Ragna - I have a moveset planned for him too
Nightmare - would be pretty beast, but what about the transforming in the FS?
Sub-Zero - This or Scorpion

!
Vergil - Would be pretty cool to have a fight with him and Dante
MvC3 Zero - Again, SDo0m is doing this next.
Etna - This dude was doing this, but he said he was "too lazy but it's not dead" No progress, I'm sure it's dead so you can take some ideas from there.
Psychryphias - Something original would be awesome! Especially from Aafyre ^_^