I like the idea of a Wild West setting. That is something familiar which has perhaps not been explored much in D&D. If it were to match up races to cultures as a starting point, I could see humans, dwarves and halflings being the "old world" races, with elves/wild elves being the Native Americans, eladrin/high elves being the Aztecs, tieflings as the priest class of the Aztecs, servants of an evil sun god (who seem to almost always be good in D&D), and perhaps deva/aasimar being outcasts from a more prosperous time in that society. Dragonborn would be the Mayans, with the distinction between couatl and dragons blurred, and gnomes would be the Incans. Oh, and half-elves would be Californians and Mexicans, as in Eberron their own group. Something I think I would do though is keep all other humanoid and monstrous humanoid races out of the setting, and focus on actual monsters for threats beyond hostile peoples.