Well, more to the point, the best-case scenario for M/K "happily ever after" still has them on the run from the aliens and the Cancerman types, which doesn't ever qualify as happy fluffy fic, in my opinion. But if you like M/K, and you haven't read Torch's M/K, I would go now and do so immediately: http://www.strangeplaces.net/torch/xfstories.html (There may even be some happy fic there, among the shorter pieces). I'd recommend her recommendations, too--I can't even tell you how many great authors and fandoms I discovered through her recs: http://www.strangeplaces.net/torch/recommend.html