I don't particularly expect it to happen, but I'd be pretty strongly in favor of their mixing up the dancer/pro pairings regardless of whether or not they allow prior winners onto the season.
If you want a 'best' of the best thing, then it really would be fair only to have, at most, prior finalists. Otherwise, it's not showing you can beat the best to stay the best, it's just showing you can pound on people you already beat once before (or who lost badly to someone else.) Not to mention that some of it's not about ability--I doubt anyone's gotten MORE popular than when they couldn't get votes last time. So it's not so much "I'm still working hard to stay on top," it's "I'm still more gifted and popular with a more popular pro than you." Forcing them to change partners even in cases where scheduling wouldn't require it and not picking returnees who got badly trounced last time (we don't need to see Buzz, Cloris, Master P, etc. again, or even Kenny-besides, let's leave him on Dance Center!) would go farther to making it a 'best of the best' rather than 'anyone we could get.'