I could see them doing, like... event match packages. Or break the targets. 10 new event matches for $2.99.
I don't think they have the balls to do new character DLC. Probably not even stages, either. Even then, there is SO much potential for DLC in Smash. Probably more than any other game ever made. You have:
-alternate costumes
-music packs
-additional single player modes
-new taunts
-new announcers
-event matches
-break the targets
-board the platforms (BRING IT BACK)
and the list goes on. I really hope Nintendo does support Smash 4 with legit DLC in the future. Say... a stage/music from the new Wii U Zelda game. I know I'd buy that in a heart beat, and it would be a shame if we have to wait another 6 years to see that game incorporated into a Smash Bros game.