Fomorians at the Alamo
(Old West, Religion, Celtic Myth)
This campaign has the role of the Mexicans at the Siege of the Alamo taken by the Fomorians. Within the small fort, Texans and Faery Folk fight side by side. Christian preachers are having a bit of problem with the pagan Tuatha de Danann, but so far tensions are low. At least until the siege.
I think this could be a fun one-shot. Coming up with a reason why the Celtics are present might be dicey, or just don't explain, just play.
Merlin and the Mormons
(Illuminati University, Old West, Camelot)
Immortal Merlin subverts the nascent Brigham Young University into a Center for Weird Studies in 1870s Provo, Utah. The young reborn Arthur learns gunslinging and state craft to rebuild the territory to prevent further onslaught from bandits.
Perhaps this could be a one-on-one with a player for Arthur and the GM. Or you could go with young 'knights' going to classes to learn shooting, time travel, and psychomancy to get the skills to become the Gunslingers of the Round Table, righting wrongs and searching for the Holy Grail, or The Golden Cup of El Dorado.