I really want to see Captain Hart/Suzie Costello in any capacity now, holy crap.

And if we're on the subject of crossfandom companions, I want to nominate the Fighting Agathons from Battlestar Galactica, because the Doctor really needs more kickass married couples in his life who won't stand for his repressed longing. Also because facing the cybermen would be hilariously problematic. I can see it now: "For the last time, Karl, they're not people. Er, no offense, Sharon. You know what I mean! Seriously, stop that. I can't take you anywhere!"

I think I'm going to stop before I accidentally write this.