Moffat began writing the fifth season with Tennant in mind, and though Tennant intended to leave with RTD, he talked with Moffat about the plans for the fifth season. Moffat had already written "The Time of Angels," and Tennant considered staying. (In DWM Moffat says Tennant turned down the fifth season during the filming of "Silence in the Library," but The Writer's Tale: The Final Chapter places it much later than that, while Tennant was doing Hamlet between "The Next Doctor" and the specials.)
I read an interview with Moffat just yesterday where he mentioned that at least in his early scripts for the fifth season, he wrote the Doctor as a little generic and a lot Tennant-esque, but that now he writes the character for Matt Smith because he's seen how Smith approaches the role.
By and large, I think the fifth season could have worked with Tennant, and I suspect that "The Big Bang" would have served as the regeneration story (with regeneration coming with the universe's reboot). I think we'd have missed out on things like "fish fingers and custard" and "bad, bad beans," and the ending of "Flesh and Stone" would have needed a rework (Tennant's Doctor wasn't that bloody clueless around girls, after all).