Well, an adventure mode to start, as well as the removal of Smash Tour in the next installment (I did not find it fun in the least, I only did it for Pac-Land). The SSE was one of the best parts of Brawl, and not including a new adventure just because "people will upload the cutscenes" is not a good excuse - uploading cutscenes has happened in many video games, not just Brawl (you've seen how many people have uploaded cutscenes from South Park: The Stick of Truth, right?). That's like saying a sports league needs to reduce the amount of teams it sends to its playoffs because of the amount of weak teams that get in (when in fact, in one case, expansion is needed).
Also, all past veterans should return, even if not part of the initial release - meaning some veterans would be DLC. A total of 61 characters have been playable in Smash over the years (including all three styles of the Mii Fighter, plus characters that could previously change between different forms mid-match, counting each form separately, but not counting alternate skins like Alph or the Koopalings).
To address technical issues with the 3DS, if we have 2 versions like with SSB4, having version-exclusive characters should not be a problem. I would say five exclusive characters per version, all from lesser-known titles, with the Wii U version's characters coming from home console games (one of them being the Ice Climbers) and the 3DS version's characters from handheld games. That's if the new games are even on the Wii U and 3DS.
Also, one big new feature involving music - the ability to use custom music (like MP3s) that is on an SD card. I like to play my favorite classic metal tracks when I Smash, but now I need to play the songs from my computer.
Minigames would also receive a major overhaul. Classic minigames like the old Break the Targets (as well as the current Target Smash), Board the Platforms, and Race to the Finish should return, as well as Home Run Contest. I'd also like to see new minigames, for example, a Swim Race minigame that expands on the game's swimming mechanic, where you can race against others (humans or CPU), or do a time trial. As well, I'd like character-specific minigames, meaning each character would get a minigame exclusive to them. For instance, Pikachu could navigate through a maze and when he gets out, he finds a piece of cheese at the end (a reference to something done with real-life mice).