its not at all clear what you do in a campaign length Western game
In a way, Western campaigns are a lot like Horror, Sci-Fi or Superheroic campaigns in that you generally fight the same kinds of foes over and over again. Occasionally, you'll get an oddball opponent, but for the most part, its the same guys, different days.
What differs is terrain and tactics. An Indian attack on a wagon train on the open plains differs from a raid on a farm, a Fort or a full-blown tribe on the warpath. Outlaws rustling cattle is different from them robbing a bank, a train, or even a shootout at their hideout. Range wars vary upon what's being fought over: grazing, water or the land the railroad will be passing through.
Of course, if you go into Fantasy Western or Alt-Hist tropes, you can have all kinds of oddball things thrown in. The
Deadlands RPG and Kurt R.A. Giabastiani's
Fallen Cloud novels spring immediately to mind, as do the shows
Wild, Wild West, Adventures of Briscoe County Jr., and
Kung Fu. Going further afield, you could arrange to have the events of
S3 Expedition to the Barrier Peaks re-set in the Rockies, or lift stuff from
RIFTS Spirit West or even
CoC.
Heck, even as far back as the 1960's & 70's, you had fiction depicting Cowboys vs Dinosaurs, Dragons, Werewolves or Undead.
For more info & ideas, pass by
this thread.