Americana (DnD Campaign Setting)/Adding On
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To quote the original author, Americana "is taking the general mythologies of different areas of the country and porting them into the game as flavor." That combined with 3.5 Edition D&D and a slightly steampunk setting gives you the setting as it is, and should give you a good idea of where to start from there.
If you want to help with the setting, feel free to drop me a line at firstname.lastname@example.org: I'd love to talk all about it!
Advice For Working in the Setting
American folklore is no less expansive than Europe's--the main difference is that it tends to be more focused on people instead of mythical creatures, and magic is at best minimal. People do things that might be considered magical--Paul Bunyan cutting the Grand Canyon, Cap'n Stormalong coating Dover's cliffs white, Pecos Bill painting the Painted Desert--but these are rarely considered magical, they're just how it happened.
So the first piece of advice is roll with it. If something sounds too absurd to be true when you write it, it's probably not. Nazis in the moon? Sure! But make it fit. Make it fit is the second part. Things should not feel out of place.
Reality is stranger than fiction is the third piece of advice. Mike Fink sounds like a cartoon villain on the NPC page but all of the things he's actually said to have done are things the real Mike Fink was said to have done. If you want a character for a role, there's every chance someone has been made for the role already in American fiction. That's not meant to dissuade you from making a new character, mind! But if you're coming up dry, pore through American folklore. Folk songs. Local legends. Maybe ask your local librarian or history museum. This is the beauty of the setting--since it is reality viewed through a tinted funhouse lens, you can use real things as much as you like.
Go with what you know, if you live in America--or if you don't! If you live in Germany and you have only a passing knowledge of the US, that might be even better. Make use of what we have here and apply what little or nothing you know to tint the window, that's a perfectly valid way to interpret it! If you do live in America, you know more about your local lore and quirks than I do--make the most of what you know!