P.S. Seems you folks're very strict in terms of plausibility. For goodness's sake, it's fiction! (It is true that implausible scenarios annoy me sometimes, but I'll ignore the implausibility if the story is, like, really really good.)
True, it's fiction, but alternate history is a very specific genre that panders to a very specific group of people, namely people interested in history, so we want to see what our world could look like.
There is a reason why some alternate historians don't like involving aliens and time travel in alternate history, because those two things have nothing to do with history. If you want to make stories involving aliens or time travel, that's fine, but don't call it alternate history.
Case in point: Does anyone remember AHH's book, the Atlantropa Articles? While it was a good book, I would hardly call it 'alternate history'. It is more of a post-apocalyptic story involving Nazis.
Isn't alternate history a subcategory of scifi?
I like to think of alternate history and science fiction as two separate genres.