Agree with the above. No danger they'll 'patch' in new characters, that'd be a strange thing to do and probably wouldn't be too well recieved by the hardcore of fans who play regularly and to high standard.
As for releasing "SF4 II - The next Batch" or whatever they'll call it, although this is traditionally what happens with fighting games and particularly Street Fighter games, I'm not quite so sure it'll happen

I think SF4 has been a much bigger scale release than any previous generations of SF4. There have been adverts on TV, posters up in every game shop on the high street and the internet means everyone gets to see the huge (well deserved) review scores and so this games has sold masses, comprable to any other big release.
Problem we get to, is that SF4 appears a very simple concept yet playing can be exceptionally deep. Most who are new to the game will play online, get destroyed, get fed up, not play anymore and in a years time SF4 will be back to being a niche game.
If in a year or 18 months or even two years Capcom release "SF4 II - The next Batch" I don't think it will be sufficiently different (I hope it won't be too different!!) to entice large numbers of people to buy. The only takers are likely to be the hardcore still playing SF4 religiously two years after release.
I think what my above rambling is trying to say, is thatI don't think there's enough commercial interest to make it viable.........
(I'd love to be wrong though)