Well, Cameron wrote Avatar in like '94/'95, so the broad concept for that predates Animorphs.
Hm, I don't know if anything's coming to mind for stuff directly inspired by it, though I guess a lot of the modern young adult fiction stuff shares a lot of similarities and chances are the authors might be familiar with it.
I watched the 1993 Body Snatchers remake a while back though. Way better than the recent Nicole Kidman/Daniel Craig version, and while the specifics of it are pretty different to Animorphs, it really, really frickin' really shares the same paranoid tone/vibe. And for that it's just awesome, about the most Animorphs-y thing I've seen. And the original Invasion of the Body Snatchers (and I suppose the remake which was pretty fresh and recent when Animorphs started) have been said by K.A. to be big inspirations for the series.