I'll just use myself for an example since there's some minifigs that I have that I haven't found a use for.... until recently. Since I already have 33 Ancient factions (Kahilawayans not included) in the world and started having problems choosing other historic and even mythic civilizations of Atlantis and the Rama Empire (Hindu legends)... I thought, why not create new totally fantasy or even conjecture based factions on other worlds.
I've already changed my Kahilawayan factions from being High-Medieval to spacefaring last year. So I thought, why not give them "Lang'it-Ba'kud" (HeavenGates)... better known as StarGates, and visit other worlds.
I think I've found a use for all those bizzare looking flesh heads. They can be alien civilizations on a Medieval technological level. The Kahilawayans can visit them long ago, and perhaps be their "teachers"
This'll sound airheaded, but I just came up with this idea a few days ago despite being a StarGate fan for several years!

If you have a Fantasy setting, you could link your current earthbound Castle guys with the other-world castle factions via "magical portal", a magic version of the science-fiction stargate. Im not sure how the pure-history guys will even took the route I took. Going for a mix of history and sci-fi.
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EDIT:
I have upgraded the idea into a community project.
The "SkyGate" AFOL community project