hmm... Interesting. I think perhaps the ocarina of time may be a good one. Maybe with the ability to change the level mid battle, and stuns all characters for a second. I think that would be pretty damn cool.
Maybe also, the hammer from Project H.A.M.M.E.R, that could stun enemies as it flatens them out (complete with cartoon style pancaked characters).
I'm also thinking that there will be items based around other characters like Snake and Pitt.
Most likely. That's wut they did in SSBM. The bunnyhood etc. etc. were from LoZ and the rey gun and wut not from Starfox? Bomb-ombs & etc. from mario. They'll do it again in Brawl would be my guess.