4. The Doctor's Character Changes
The David Tennant specials were a mixed bag that exemplified the Davies era. There was traditionally Who-ish concepts realised disappointingly (Planet Of The Dead), an assured, different one (The Waters Of Mars) and an overly bombastic finale that threw everything in the mixing pot with only some of it working (The End Of Time). The best of the lot, The Waters Of Mars introduced two really interesting avenues to be explored that were cut short by the end of the episode. The first was something mostly absent from the Davies stories messing with fixed events. We got to see a Doctor finally disregarding the rules of the destroyed Time Lords and changing time. Yes, time does correct his actions much to his disdain, but there was a major character change here that would lent itself to more stories. The second is something that we did finally see with Matt Smith in Series 6; a Doctor who was ready to die. After the previous example had been corrected, Ten expressed a desire to die that was absent when we saw his next in his final adventure. It's a shame this change was reverted, but even though Tennant is returning for the anniversary special this is an avenue that was never explored beyond one scene I guess Davies wanted to look at these elements of the character, but given the short time left he rushed the job a bit. The themes are given focus, but not as much as youd imagine they warrant.
Potential Solution: Its all off-screen.