OMG Doctor Who is wonderful for crossovers (cough, I would say that, wouldn't I). But really, you don't even need to think too much of how the two universes work together - I mean, it's DW! He can just show up there :D

There can never be enough Doctor Who/ Big Bang Theory crossovers. Also, DW/ Labyrinth. And I have to say that I think DW/Labyrinth would probably work best with Moffat's Who. It's all fairytale logic!