I've thought about this may times before. You know how once melee was created, we had to wait until the Wii for the next Smash Bros. game? I believe the reason that they did this was because they felt that if they released another Smash Bros. game for the same hardware, it would basically just be an expansion (new characters, new levels, etc.).
So, I was thinking, just hypothetically, what if a couple of years after Brawl came out, they released a sort of downloadable expansion pack? It wouldn't just be characters in it, there would also be new stages and other stuff. It would all be released at once and all the characters would be balanced. If you didn't have access to the internet, they could sell it on CD. I thinkit would make everyone happy. It probably wouldn't happen, but who knows?