Random thought here: I wonder if part of the reason for so many crossovers is Highlander's relatively small core cast? The episodes were a revolving door of Immies of the week. If you look at other fandoms, say, Firefly, you have a lot more people to play with. So moving Methos and Duncan to the future, where they get to interact with all those Browncoats means being able to write a story without having to create original characters.
I know that's a part of why I have so many original characters when I write Highlander. It just seems unrealistic to me, as someone who does not ship D/M, that Duncan, Methos, and Joe, would never socialize with other people, date other people, for year after year. Amanda flits in and out, of course :) A lot of the time the 'crossover' aspect it just a setting for the Highlander characters. I may write a something that's technically a xover with Star Trek, but like in my last story, there wasn't a single ST character. It was just a way to utilize the mirror universe concept and be able to play with Kronos [g]