My guess on playable characters is several of the Assist Trophies from Brawl will be playable characters this time. Since Sakarai said some of the Assist Trophies were based off of heavily requested characters that didn't make the final cut, I'd imagine several of them will make the cut this time.
For one, I'd say Little Mac is 100% guaranteed this time. He was heavily requested for Brawl, got an Assist Trophy, and his series was recently revived and it sold quite well. So when all of this is put together, he's the one character I expect to be 100% in. Hell, I wouldn't be surprised if he's included in the first trailer that will hopefully be shown at E3.