So basically, we are trying to create a streamlined Brawl product for to be released a few years later after Brawl's success that focuses mainly on multiplayer aspect of the game, and less on the single player.
Well, here is what you do:
Elminate Subspace Emissary from the game.
Remove the ability to costum Create levels, however keep the ability to have those levels downloaded each day and play. (This may sound weird, but it is more about keeping both games compatiable with features, but also keep Brawl the prefered game.)
Remove trophies, Stickers and other elements.
Keep All Star, Classic, and Event modes for Single player options, and add in completely new and different Event Battles to create a new experience for this version.
Add in more break the target levels, and bring back Board the Platform. Make both playable online in both competive and co-operative modes.
I would not add in Nintendo 64 levels, instead I would create 10-12 new levels (Again, reason being is to create a difference between Brawl and this new game.) These stages need to be really good...stages that have been neglected in the franchise. Bowsers Castle is the first on the list.
Finally the game needs to have atleast 5 new characters, beyond clones. Krystal, Mega Man, Tails, Drill Dozer, Midna and Wolf Link, Daisy. Also, I would give Gannondorf and Toon Link a completely new moveset.
Stll, even with all these changes I dunno if I would get the game...it would have to be budget priced.