9. David Tennant Stayed On For Series 5
When preparation began for the beginning of Steven Moffats reign as Doctor Whos head writer, whether or not David Tennant would be leaving the Tardis remained to be seen. Not knowing whether or not Tennant would stay, Moffatt made rough plans for both a last series with the Tenth Doctor and a first series with the Eleventh Doctor. Had Tennant stayed, the first episode of Series 5 would begin, as The Eleventh Hour did, with the Tardis crashing into Amy Ponds garden and the Doctor meeting Amy as a child. But the change between the two versions is that the Tenth Doctor would be injured from the crash when he met Amy but when he reappeared to her later in her life, he would be uninjured and have no memory of meeting her. Series 5 would progress, presumably in a similar way to how it did with the Eleventh Doctor, but would end with the revelation that the Doctor we saw at the start of the first episode was from the series finale where he was injured and about to Regenerate, bookending Tennants final series in a typical Moffat time loop. The wider effect of Tennant staying would presumably be that Series 5 wouldnt be as big a revamp as it eventually was. The Tardis interior would still be changed at some point as a new set suitable for HD filming was needed but since Tennants Doctor was a holdover from the Russell T. Davies years, the tone would probably be closer to that Series 1 4 than to Series 5 as we know it in order to retain a sense of continuity until the Tenth Doctor departed.