I prefer Melee to Brawl too, but that said, I think the SSE is an awesome adventure mode, and I sincerely hope they offer a 'sequel' or something for the new SSB game. I don't know about having the bosses as playable characters... maybe a special mode where ONLY the bosses are allowed to be chosen as if to see which one is the most badass, etc. That said, however, I would love for Tabuu to be a playable character, maybe even incorporated into the story. Of course his moves can't all be reproduced if that's going to be the case but then simply making him as a playable fighter, he'd get a new set of moves that would make him 'fair'.
I would like to see a return of the tournament mode from Melee because I loved that! Also keep the 300+ trophies to collect (lets aim for 500+ this time). I didn't like the retro demos, I found them really pointless and felt the space could've been used for MORE trophies
I also loved the stage editor and felt that more variety and options could be used! Get rid of clone moves and PLEASE no more Link alternatives (ie young link / toon link).