But it is weird to see almost exclusively crossovers lately.
Isn't it? I've always loved crossovers and have written them/read them for about 10 years (albeit with a writing break). When I first started looking, crossovers with Highlander were a rarity, it tickles me now that they aren't. I think the longevity of the fandom, crossed or not, is because of the way the basic premise of the Highlander universe touches something deep within us. That can be adapted and applied to just about everything else - you have a particular 'future' fandom or era of history that fascinates you and you can pretty much guarantee that immortals will be there!
I find that if the fic is written well I can follow it with even just a vague knowledge of the other fandom - SG1/SGA, and NCIS fics crossed with Highlander have all been examples of this. The latest one to do this is the Dresden Files (though that was a Torchwood cross) to the extent where it's taken me about 3 months from first hearing about the Dresden files in a crossover to having read all the books (on the last one now) and waiting for the dvds to arrive!