Methos is the coolest character on Highlander: The Series. HL is the show that's crossable with just about EVERYTHING. It spans timelines so anything that takes place in our world-- in any century-- is fair game for a HL crossover. As is any country or location. It also lends itself well to stories about supernatural powers, like The X Files, or Buffy, or even comics like XMen.

It would be hard to cross it with ATLA seeing as ATLA takes place in a mystical fantasy world, but still, it wouldn't hurt the fic at this point.