I'd like Chell (Portal and Portal 2 protagonist) to be on SMBC, I know that the game isn't old but it would be cool to use portals, you could drop enemies into another ones or into falls, it would be cool
While interesting, SMBC is a collaboration of classic NES-era (or styled) characters. I do admit to enjoy having fun, thinking with portals, but there would be a royal pain of logistics to how it would work in the SMB environment.
Somebody's actually making a Mario version of Portal, giving Mario and Luigi portal guns. It's called Mari0 and should be released in about two weeks.