I ran a fantasy America based on the resonances between Celtic/Grail and Mormon and Iroquois myth. I think it was pretty effective. I think there are great possibilities for an alternate fantasy/medieval version of the Americas.
I think that the best strategy for producing a fantasy America is to look for the similarities rather than the differences between American and other myth systems, cultures and governance structures, rather than going for an American 'feel.'
I think that the best strategy for producing a fantasy America is to look for the similarities rather than the differences between American and other myth systems, cultures and governance structures, rather than going for an American 'feel.'