io9 has a neat little rundown of the mysteries the finale left open (spoilers of course). The writer, by his own admission, reads WAY too much Key to Time into the whole affair, but there are some interesting theories along the way.
[spoiler]Prevailing theory is that the Woman is the Doctor's mother, and that he looked from Donna's mother to her to indicate the relationship between them. The Susan theory is also represented, suggesting that he looked from WILF to Donna.[/spoiler]
I also quite like the idea that the Ood are evolving into a new race of Time Lords.
At any rate, Davies left Moffat a lot of toys to play with, and I expect he'll pick a few up yet. At any rate, Silence in the Library/Forest of the Dead made more hints at the Doctor's pre-Doctor personal life than pretty much anything else ever has, so he'd be a reasonable pick if we're going to find out more about [spoiler]the Doctor's mother[/spoiler].
Onward and upward:
Bleeding Cool takes a look at a few key frames from the trailer for season 5. And makes an
observation about Smith.
At any rate, this certainly isn't the last we've seen of Tennant, or at least heard of him. Radio series, allons-y!
Tennant already pointed out at Comic-Con that the 50th Anniversary is in '13 and would be a logical point for him to return. He quickly backpedaled and said there were no plans to that effect, but I'd really give it pretty good odds. In fact I wouldn't be surprised if all the surviving Doctors were onboard for a return, given that Tom Baker's finally started doing radio shows, making Eccleston the last holdout and for God's sake he appeared in The Dark is Rising and GI Joe so it's not as if he's really particular about his roles.
I read a Moffat interview awhile back where he said he wasn't going to bring back previous Doctors unless there was a good plot reason to do so, but I'm sure he could cook one up. Time Crash made for a good, what, eight minutes with one previous Doctor; I'm sure he could cook up a good 90 minutes with 7 of them.
Plus, certain things are just obligatory. He's already doing a Dalek episode, after all.
I say with no reservation that Tennent is my favorite Doctor, even surpassing Tom Baker at this point.
Hm. I'd say mine probably goes Baker > Tennant > Eccleston > Pertwee > McCoy > Troughton > Davison > Hartnell > other Baker. (Can't really make a call on McGann from his one appearance; I haven't checked out his radio serials, aside from the Shada remake.) But there's wiggle room.