I don't know how much this is done here, so if it's not proper, feel free to delete, but I've got a couple Doctor Who crossovers I'd love to see and don't quite have the motivation to write myself.
Doctor Who/Lost: Partly this is because I've been saying more and more as I watch this season, "You know, if the Doctor showed up here, he'd have everything sorted within the hour." But after reading part of
an article speculating on the show after the most recent episode (full disclosure: I want to read the whole thing but have only actually read the paragraphs on that page about "the timeline math"), I really want to have the Doctor (Ten, probably) show up and...fix things. I mean, messing around with Time? Not on his watch. It'd have to be an AU for Lost, obviously.
Doctor Who/Star Wars: My knowledge of canon here doesn't extend beyond the movies, aside from a very small handful of EU books and what I've gleaned from starwars.com and Wikipedia.
That said, when I did find out that Jacen Solo had gone over to the Dark Side, I found it hugely depressing and was glad I'd never gotten into the EU canon more than I had; if I was ever really in the Star Wars fandom, that would pretty much be enough to get me out. And then I found out that not only had Jacen become a Sith, he'd become so powerful he was somehow manipulating time itself. Obvious crossover need: the Doctor (probably Ten again, maybe Nine) shows up to knock some sense into Jacen and get him back on the right side of things, if he can, or at least stop him messing around with time, if he can't. Again: nobody messes with time on the Doctor's watch. Another AU, obviously (bonus points if the Doctor shows up soon enough to save Mara). Like I said, I can't write this one because I don't have enough canon knowledge, like what the exact sequence of events was in the Star Wars universe, but it'd be a great story.