I just keep thinking about 10b with all the Doctor's memories living in a parallel dimension - a bit of pseudo-science and the walls between worlds can come down again and out pops Rose and 10b; this probably won't happen of course, but it allows for an easy two doctors story in the future without having to worry about walking all over the Doctor's personal timestreams etc. so 10 hasn't actually gone...from a certain point of view.![]()
10b always seemed to be designed as Tennant's way to reappear on the series as much as he would like to in the future. If I understood correctly, 10b will age at the same rate a human does; so 10b will look like Tennant as Tennant grows older.