honestly, the only characters i totally expect to return are the classic 12, some of which have already been confirmed (yoshi, luigi, capt. falcon, ness and jigglypuff have not yet). outside of those 12, i would bet on zelda, peach, wario, another pokemon & ganondorf (with a moveset that differentiates him more from capt. falcon) though. falco has a good chance, but i expect mr. game & watch will leave. waluigi might be added, and olimar and diddy kong might return as well. ice climbers have been specifically mentioned in the dev. diary, but they mentioned they're having problems with them on the 3DS (not that it won't be resolved).
as for 3rd party, i think sonic deserves to return and pac-man seems like the obvious choice from namco. btw, i am ecstatic for mega man (see my avatar). i would love simon belmont if they can get the rights from konami, but castlevania seems to be a pretty dark game for such a bright set of characters. i don't expect more than 3 or 4 third party characters though, as the focus should be on nintendo franchises.
also, remember that the devs have mentioned they don't want to increase the roster and keep the roster balanced and unique.