Billie Piper and David Tennant are returning for the 50th anniversary special,confirmed by the BBC here. And there was much rejoicing online. Well, much rejoicing tempered with concerns about Moffat writing Rose. On the other hand...I have a feeling I'll be flooded in hate once I post this on Tumblr.
None of what follows is a judgment on Tennant or Piper. They're fine actors, with outstanding roles outside of Doctor Who as well as their significance within the fandom. I may not like certain aspects of the characters they played, but I have nothing against them. Also, I have seen all but one season of Classic Who, so what follows is said as a fan of ALL the Doctors, not just NuWho.
I have concerns about the length and number of the specials, though I won't go into them here. In my opinion, the purpose of a 50th anniversary special is to commemorate the entire history of the show, not just the revived series. Therefore, the characters and monsters should be iconic--the things people think of when they say "Doctor Who," not just one-off enemies, no matter how well-liked. For Moffat's era, that would be Ponds and Weeping Angels.
Therefore, I think that Sarah Jane Smith or the Brigadier would have been the most appropriate characters for a return--which is impossible, sadly. Both appeared with multiple Doctors and spent multiple seasons in the show. I'm not sure who I'd pick as a runner-up, but I have dramatic issues with Rose.
The announcement didn't say that Piper and Tennant are reprising their roles as Rose and Ten(Two?)--I think it'd be cool if they weren't, but I'm going with the obvious assumption. So either we have Rose and Ten from season two, or Rose and TenToo from the alternate universe. I'm not thrilled with either option, but again, the latter seems more likely.
I didn't like Rose's return in Journey's End--it cheapened her departure in Doomsday. I mean, we're told both universes could collapse if they were breached, so how did Rose make it back? The stars were going out, but I would think that would only increase the problem, not let her come through. Flashbacks and clips are one thing--I was fine with the cameos in Let's Kill Hitler, even the End of Time moment--but her story has reached a satisfactory conclusion. So has the Ponds. I love them, I'd love to see them again, but the Doctor can't. Period.