I would recommend throwing in a CoC vibe; the historical West has very limited scenario fodder other than law enforcement and the strife between settler and Native American through the 1870s, and the later union strife and fence/open range issues.
I'm puzzled by this given the breadth of stories within the western genre beginning with dime novels in the 19th century and then moving on to television, motion pictures, and even radio in the 20th century. We have more than a century where westerns dominated popular media and most of those stories didn't involve the supernatural. Just looking at a synopsis of episodes from season 1 of
Rawhide will give you plenty of ideas that don't involve strife with Native Americans, unions, or the range wars. Though, uh, some of those episodes certain touch on those issues.
Here are some campaign ideas that don't involve the supernatural at all:
The PCs have just been released from prison. Now they're headed to Texas to take on Brisket Bill, their former leader who betrayed them after their last train robbery and made off with a fortune in gold.
The PCs are all "upstanding" citizens of Cripple Creek, a mining boomtown in the Rocky Mountains of Colorado, and you need the town to prosper so that you can prosper. That means keeping the peace and make sure everything runs smoothly. You characters might be part of the local constabulary, editor of the local newspaper, owner of a dry goods store, or perhaps even a member of the city council.
The PCs are responsible for scouting and and securing the best sites for the Union Rail Road to lay down their tracks. Which towns will the rail road go through? That's up to you.
Now that I write this, I've got to admit that I can count on one hand the number of campaigns I've run that didn't involve some aspect of the supernatural or the fantastical.