DLC in Smash, or post-release content in general isn't impossibly unlikely. Nearly so if the team does disband post-release, but if that's not the case, take a look at Ninty's recent history. Super Weegee U just was that: one gigantic DLC. Mario Golf 3DS is gonna have the unthinkable: Day one DLC (... it is day one, right?). Strange things are afoot in Ninty's offices.
... Not that I'm tryin' to contradict myself from before. I sure as shit don't think Smash 4 will have post-release content, or even balancing patches (bar to fix game-breaking glitches).
I'm speakin' from a Fire Emblem (and coincidentally, a competitive Smash) point of view, mind ya.