There are a few theories around Doctor Who and Buffy being linked, or sharing a universe, with a couple of characters actually speculated to be Time Lords. One is Giles, due to his belief that he was dying at the end of season 6, which was actually a regeneration, and he somehow managed to keep his appearance. Others include Buffy herself (she dies a lot), or even just the whole Slayer line, which means that its just one Time Lord who has constantly kept regenerating. Obviously it's not going to be true, but personally I'd like to see Anthony Head as The Doctor. There's also the matter of them being set in the same universe, which comes from the fact that the Tenth Doctor and Rose appear in a Buffy comic, the two iterations of the Master, and the appearance of Anthony Head in an episode of Doctor Who, with Lassar stealing his appearance.