Yellowstone is a very addictive place, so it isn't at all surprising to me that you'd want to go back.

A couple other areas that might fit well into your next trip that could provide something quite different than heading down into Utah.

I think a loop that takes you east to South Dakota would be one good option. You could check out Badlands National Park, as well as the Black Hills - with places like Mount Rushmore, Crazy Horse, Jewel and Wind Caves, and Custer State Park. In between, you'd also have Devils Tower in Wyoming, and Little Bighorn Battlefield in Montana. You could also take two different routes into and out of Yellowstone - Going over Beartooth Pass one way, and perhaps through Cody the other. Billings or Rapid City could be good cities for flights. This would certainly play well with your interest in Native American culture.

You could also look at going north from Yellowstone. Glacier National Park would be a good option, and from there, you might look at continuing across the border, towards some of their great parks of the Canadian Rockies, like Banff and Jasper. These are all very scenic areas that would make for a very nice couple weeks.