-One of his own enemies from the past.
Especially when it’s pretty much been confirmed. (See bellow for why. I don’t want to type it out again.) As said, I believe it’ll be the Master, but it could be anyone. Or, taking a leaf out of the original Shadow Proclamation’s book, everyone. (See ‘The Writers Tale’, by Russell T Davis. I know it’s £30, but it’s worthwhile.)
-The entire universe at stake.
David Tennant’s Doctor is the most popular since Tom Bakers, and how did he go out? By toppling radio telescope whilst battling the Master, with the entire universe at risk of falling into his clutches. Hartnell just collapsed in the TARDIS while Colin Baker hit his head on a Bicycle pump (*Runs*) If Tennant goes out in either of these ways, they’ll be a riot.
-The return of an older companion
Probably Martha or Jack. I really hope it’s not Rose, and Donna will literally burst in a ball of flames if she remembers he days as a Time-Traveller. Maybe someone like Astrid (Hopefully not) or Christina, too. Elizabeth Sladen said that Sarah Jane probably won’t be popping up, as she said in DWM #397. More on this at a later date.