It is truly amazing that a game like Smash literally is limitless on characters because people want their favorite characters from the past included...not to mention any favorite new characters.
I think a Port is the obvious answer, adding in all the DLC into one package would be a great incentive for gamers, plus I forgot about the idea of characters that were not included in the new one.
Snake, Ice Climbers, Classic Zelda (I would love a new model of Zelda and Sheik to become Toon Zelda and Toon Sheik then bring back the old one) Pokemon Trainer, Wolf, Add in Inklings and a new crazy 3rd party character ( Bomberman) and you have a great package alone.
But wait, what about the exclusive modes? Bringing back all the modes from previous games: Race to the Finish, Break the Target, Adventure Mode (Don't focus on a level for each character, just a level to represent each franchise)
Add all exclusive modes from each game, and flesh them out if you need to. This could easily be a full priced game.