I'm pretty much thinking all of the poll choices aside from SSE and possibly boss mode.
Event Mode is probably a given. It's a liked-enough mode and it fun. A Melee-style adventure mode would greatly increase the single-player replayability, which is super big for me. I really wished that Brawl had had one, SSE was definitely not the same/as good. I'm hoping we have a mode like that in SSB4, a lot.
Classic Mode may be longer than the 3DS version, I'm not sure though. In all likelihood it'll be the same length.
Boss Mode is the one I'm unsure of. There may be bosses in the game, but I'm not certain on this at all. The Boss mode in Brawl was a direct spinoff of SSE. I'd like to see it return, but I wouldn't be at all surprised if it didn't. But I wouldn't want to add stage bosses/sacrifice K. Rool or Ridley's potential playability for the sake of an extra boss mode.