I've always wondered how the sixth series would have been different had David continued on as the Tenth Doctor and Moffat introduced Amy and Rory with him.
Last summer, on my blog, I pondered
the fifth season with David Tennant. I can
almost make it work in my mind, as Moffat tends to write the eleventh Doctor with very tenth Doctor-ish lines, even as late as "A Good Man Goes to War." I don't know what "The Eleventh Hour" would have been like with Tennant, though we wouldn't have had "Fish fingers and custard," obviously. I think that much of the season would have been similar, and I can see how "The Big Bang" could have been a regeneration story, if Tennant decided to leave at that point.
The major difference, obviously, is that casting would have been different. There's no guarantee that Gillan and Darvill would have been cast as Amy and Rory in a fifth season with Tennant, as both were cast due to their rapport with Smith. I even doubt that Amy would have been Scottish; Tennant and Barrowman have both said that they can't talk in their Scottish accent to one another because then they'll both lose their acting accents, and a Scottish Amy could have broken Tennant's concentration.
There is a problem, though. Davies' tenth Doctor had a definite emotional arc that stemmed from being the Last of the Time Lords. The eleventh Doctor doesn't have the same emotional cloud of gloom hanging over him; it's as if the regeneration closed off some of the emotional wounds that the tenth Doctor carried. If Tennant had continued as the Doctor into Moffat's era, if we'd had the fifth season with Tennant instead of Smith, the season would have felt like an emotional repudiation of the weight Davies' tenth Doctor carried.
In short, I think the fifth season would have been okay with Tennant, but we'd have had a different cast, and I don't think it would have been emotionally
true to everything that had come before for the tenth Doctor.
If Tennant had stayed for the
sixth season...
Thus far, I don't see anything that wouldn't work with Tennant, except maybe for "The Doctor's Wife." And yes, I realize that Gaiman's first draft was written before Smith was even cast. It's just that Tennant's Doctor seems incapable of Smith's sense of glee.
By and large, though, replacing a Doctor isn't that unthinkable. Moffat says that, in his mind, all the Doctors are the
same; they have the same memories, they are the same man, it's just their bodies that are different. "The Girl in the Fireplace" was written with Eccleston in mind, "The Caves of Androzani" was written with Tom Baker in mind, "The Time of Angels" was written before Matt Smith was even cast. It's the actor that changes, the Doctor stays the same.
