The original idea for The Tenth Doctor's final story would have seen the TARDIS materialise on board a spacecraft carrying a family of aliens. The Doctor would sacrifice his life to prevent a radiation leak and save these ordinary, unimportant beings. Although Russell T. Davies liked the notion of the Doctor's regeneration taking place in such unremarkable circumstances - as opposed to the blockbuster events that had characterised each of his season finales - he was already concerned that viewers would consider this to be too great an anticlimax.
Russell T. Davies originally wanted to have The Doctor and The Master swap bodies. However, Davies wasn't keen to spend time during David Tennant's final story with the actor playing anybody other than the Doctor, and was also mindful that he had used a similar notion in "New Earth".
As part of the promotion for The End of Time, BBC One aired a special ident during the Christmas season in which the Doctor (David Tennant) would ride the TARDIS with reindeer like a sleigh. According to an interview with the Radio Times, Russell T. Davies credits his inspiration for the ident from A Matter of Loaf and Death (2008), who had a special Christmas ident in 2008. This ident was also adapted for BBC America.
Russell T. Davies originally planned for the Master leave an 'M' at the scenes of the murders he committed so as to provide a trail for the Doctor to follow. This was dropped when he realised that it had already been established that the Doctor would simply 'know' where to find his fellow Time Lord.