1. Host migration/better netcode- I think these are the things that can help solve the lag problem for the most part. With host migration, if the host of the game (the person with the strongest internet connection) leaves the game or is having trouble hosting it, then the server can choose a different player to host it instead of just ending the game.
2. Voice chat enabled- Self-explanatory. Comes with an option to mute desired players, and you can chat with anyone this time around.
3. Custom and Private Matches- It would work almost exactly like Special Brawl Online, except only people you, your friends on Wii U, and the people you invite can play with you. Sakurai can also take this a step further by having a contest for who can make the best custom-made game, choose the best custom-made games to become actual online modes in the online playlist.
4. Patch/Hotfix System- Basically, as you probably already know, if something's unbalanced or glitchy in the game, they fix it via online updates for the game. They'd also be used as DLC updates for when DLC is about to release
5. DLC- Characters, stages, music, maybe even a new mode if they wish
6. Ability to set up online tourneys- Just a shout out to the more competitive players, not sure how this would work my damn self
7. Ability to veto stages- A stage is randomly picked. Players in the lobby can veto it and if the majority of the people veto, then a different stage is chosen.
8. King of the Hill- Something like MK 2011 where spectators can join a match and comment on and rate it with their avatars/Miis using certain buttons or motion of the tablet (such as clapping or throwing tomatoes at the screen). There could be hidden commands too, like maybe if you press certain buttons at the same time, you can shout, "DO A BARREL ROLL!" out to the players or something along those lines.