I've written that medieval fantasy RPGs are impossible: the existence of powerful magical adventuring heroes will change the world so that it no longer looks or feels medieval. It will not be a world of knights and serfs and castles and sieges and conquering armies. So what should a GM do with this information? Most playgroups don't like being subjected to complicated world building, they want to just jump into a game with a familiar genre. A good solution, especially for American gaming groups, is one that I have been using for over a decade: Frontier Fantasy. Model your game after the conditions and stories of the American frontier.
I usually set my games in a world of log cabins, homesteaders, and small villages and forts on the edge of a rugged howling wilderness, where government is limited and people have to be self-sufficient. The environmental and cultural details are loosely based on the Appalachian Mountains in the early 1700s, because I live and grew up in the eastern USA, and most of my players did as well. If you're out west, or you are not American, then the Western will be more familiar to you, so your game would resemble a story in the Weird West genre.