I liked a few of what's on FF.net. There were a couple cool Roswell crossovers, and Maximum Ride is a good one to cross with Tobias.
I had a rough outline crossing Animorphs with a book called Dusk, which is about a girl with hawk DNA, but it got lost when my laptop died. I'd have to rewrite it.
I also want to cross it with Virals, the Kathy Reichs book. The thought-speech in both would make it easy for one group to accidentally overhear the other maybe.
I've thought about crossing it with another scifi show I loved, Tracker. Visser 3 vs Zin would be a cool fight to see. Plus, that show involved aliens taking over humans, so a yeerk trying to infest one of their races would be interesting. (same thing with SG-1 but I never watched it.)
I had an idea for a Bones crossover too, but haven't watched it much this season. However, someone did do a CSI cross I loved.
Also like the Warriors thing, I've thought of that. Or the other Erin Hunter series, Seekers, becuase it has a bear that shapeshifts into various animals and a human morph.
Merlin would be fun if the Time Matrix allowed some back-traveling. 'Cept there'd be no yeerks back then.
How about Animorphs and V? Can you imagine a Yeerk trying to infest someone only to find out they're a Visitor?
Along similar lines, there's a recent kids' book called Stuck on Earth that is so yeerk-like...slug-like alien coming to earth to take over a teenager's body, although this time it's to see if humanity's worth saving. I envisioned the alien infesting someone who already had a yeerk...
I can't do a Buffy cross...someone did, with Cassie as a slayer, but the trouble is, Buffy's mentioned as a fiction world. There'd have to be some portal stuff. Although Roswell was mentioned too as a fiction world...
Love the HL idea, btw...immortal controllers? Yikes. ALthough a vamp fandom like Moonlight or Forever Knight could be even scarier.
I could go on awhile, but I think this is it. I also got hooked on a cartoon called Dino Squad because of the shapeshifting aspect and would have loved to have crossed those two. Our group would be jealous of the dino morphs for sure.