I suspect Tennant might not want to return to the show as a new incarnation of the Doctor because he doesn't want that to contribute to his typecasting. I'm sure he'd be happy to portray Ten's role again in a special if it calls for that, but I think that'll likely be about it as far as him returning goes.
Besides, having the Doctor regenerate into a past incarnation, even if it's only physically (and thus the new Doctor having a personality different from Ten's but the same looks), would probably be way too convoluted for them to dabble in it, so I'm not really counting on it.
I'm not opposed to it, of course, though personally I'd pass on it, if only because I've closed the book on Ten(nant) a long time ago, and I'd rather not revisit all of that again and have to experience the withdrawal once more when it's over.
For me, forward is the way to go with regards to this subject.
If Capaldi leaves after next season, I'll actually be sad about it, for once, because I actually like his incarnation of the Doctor now. But that's just how it is.
If I had my pick with the new Doctor, I'd like to see him regenerate into a Time Lady. Sadly, Moffat has made his stance on that nauseatingly clear with his "joke" about the Queen, so I'm not really holding my breath... but who knows. I'd really love to see the interactions between Lady!Doctor and Missy, though.
I'm not too worried about who comes next after Capaldi. I'm sure they'll find a competent actor at the very least. But I do miss the "wow" factor that I first got from Tennant. I never really felt that with any of the others in New Who. Perhaps a little unfair of me, but... it is how I feel, nonetheless.