Wow, I would have so many of these... This is the sort of thing I've been into since I was a kid. My brother and I were home schooled, and to keep ourselves entertained that's what we would do for entertainment; at pretty young ages we created some pretty darn intricate alternate universes and roll playing them with toys or playing outside.
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If the South had won the Civil War with various timelines following (I've never read them, but I've seen that Harry Turtledove wrote a series of novels covering the same thing)
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If Vikings had unified Scandinavia before converting to Christianity and various timelines involving the empire they forged
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I'll need some time to think of more of them, but those were our favorites as kids. Those and of course our own Star Wars spin-offs, one of which was bringing Star Trek tech into the Star Wars universe (I built a Klingon Bird of Pray model and used it in our Star Wars playing), as well as using Mechs from the MechWarrior universe in Star Wars.
"You set a code to live by. I won't be wronged, I won't be insulted...I won't be laid a hand on. I don't do these things to other men, and I require the same from them."