Sakurai's probably the most forward thinking guy at Nintendo. Just look at Brawl and Uprising, two of the most feature-filled titles in Nintendo's library. And all that daily Custom Stage DLC, Spectator mode, sharing replays, and online multiplayer at a time when Nintendo had barely touched the Internet. There's a 99% chance he will do DLC, and use it extensively.
I'm pretty damn excited for DLC. For one, all those characters that were cut due to time constraints could be included. If Brawl had DLC, we would've gotten Roy, Mewtwo, Dr Mario, Dixie Kong, Toon Zelda/Sheik and Plusle and Minum. That's 41 characters! And then they could release characters at intervals, even just one per year would be sweet. Or when a new game comes out, they can have a character to promote it. Maybe third parties could even sponsor characters. Same with Mario Kart; make it a platform unto themselves, built for continued support, rather than just a typical game.
It would make so much sense, considering we probably won't get another Smash Bros until 2020. Imagine if your favourite character didn't get in, and you had to wait that long to see if they'd get in the sequel. Half of us will probably be married and have kids by then...