I don't know how game development works, and therefore don't know how long it takes to put a character into a game, but I do know that Sakurai has been open to DLC post-SSB4. So long as he puts his heart into SSB4, I would love to see him release DLC for characters that couldn't make it.
I think that if you took every character in Brawl, and tweaked some of the less fortunate ones (be it in statistics, or on moves altogether, or both, etc), then you would have one of the most solid potential Smash rosters ever. There wasn't a character in Brawl who didn't deserve it, and the only character I could see definitely getting cut is Ike, just replaced with Chrom. Lucas represents an entirely new game. Give him some more unique moves, and he's his own character. ROB and Sonic are completely unique, they just need to get tweaked and made a little more interesting. Toon Link represents a VAST chunk of Zelda titles. He isn't just there to humor the people who want a lighter and speedier Link. He's there to celebrate 25-50% of the Zelda franchise. And Jigglypuff and Lucario have both been made relevant via X/Y, thus revitalizing these two temporary relics, while Pikachu, Mewtwo, and Charizard are unforgettable Pokemon in and within themselves.
I could go on and on with defenses for various characters, as I believe that every character, whether I like them personally, or have played their game, or could care less, made it in for a reason. Sakurai didn't just put ROB, Bubbles, Tamagon, Takamaru, and Little Mac on a dart board, throw the dart, and then put ROB in Brawl. He thought about his roster, and what characters justifiably sum up Nintendo's history.
Do I think there will be cuts? I'm prepared for the worst. But Sakurai isn't just going to go "You know how I thought ROB/Lucario/Wolf/Toon Link/Jigglypuff was cool back in 2007? Well, all of the sudden, they've become the most irrelevant character on the face of this entire roster. Take them out now." It's all based on time, and I could see the lowest priorities being characters from franchises that already have characters repped. Like how the five cuts between Melee and Brawl were all from franchises with other reps (Pokemon, Zelda[sorta], Fire Emblem, and Mario). And the Zelda point is arguable. They basically retextured Young Link and reintroduced him as a Newcomer.
I'm more focused on new franchises. If we get a new franchise over a fourth rep for a game we already know about, then that's fine by me. Again, Sakurai seems slightly open to the concept of DLC.