I think this is a good time for me to bring up my personal Assist Trophy expectation and wish list!
Majora Moon - Slowly crashes down onto the playing field, crushing everyone in its wake...
Koopa Clown Car - Flies by and tips over, dumping out Mecha-Koopas that'll walk around until they are destroyed...
Ridley - No-brainer (Personally I'd rather have him playable, but I have a feeling he'll pop up here instead)...
Goomba - Think of it as the Goldeen of Assist Trophies...
Marin - Sings "Ballad of the Wind Fish" to put characters to sleep (Please, oh please...I want a remix of the song so bad...)
Metroid - Flies around and attaches to nearest character, draining energy and slowing them down...
Viruses - Lines of Viruses fall from the sky and pile up...Can crush you and will block your path...
Bullet Bill - Think Unown, but coming in horizontally and with more explosive power...
Lakitu - Floats overhead, throwing Spiked Beetles onto players...
Earthbound UFOs - Do what UFOs do best...Zap things!
Poo - PK Starstorm
Professor Oak - Throws pokeballs, which will capture players inside them for a few seconds, giving time for the Assist Trophy user to grab them and throw them off the edge!
Mr. Resetti - Comes up and buries unsuspecting players with lots of text! (Ahaha, that'd be an amusing sight...)
Pikmin - No-brainer...Lots of tiny Pikmin attack players or slow them down...
Donkey Kong (Arcade Version) - Throws barrels across the stage...
Ray MK II - I'd rather him be playable, but an Assist Trophy would be fine...
Little Mac - I have a feeling he'll be playable, but this is just in case...

And for laughs:
Sheik - Ahaha, I don't know why, but I have this itching feeling that he'll be thrown here...Just imagine the angst that'd hit forums everywhere!