A horse-pack based in NA should also include Native American stuff, which would be super cool, and that would include cultures of several tribes, mainly Southwest and Plains. Things many people associate with the Wild West such as teepees, feathered headdresses, dreamcatchers, furs, peace pipes, and "cigar-store indian" sculptures are from the Plains Indians that settled more in Northern states. (When I think SW Native Americans, I think of Navaho, Apache, and Hopi tribes and their art--nice pottery and woven materials.) Both are cool, but they are not the same.
BTW, would a Western-based horse pack be set in modern times, or a history-based pack? I like history, so the latter would be fun. Plus you could use more Native American stuff in a history-based pack, because traditional costume nowadays is limited to pow wows and tourist sites.
It could be set up as a "wild west town" that tourists from other eps could visit. They would just have to set it up that npcs only wear the costumes from that pack if they are in that zone. (Although players could change that if they wanted a whole wild west game.) I would love to see them do that with a medieval pack too, so the zone that comes with the pack would be a "Renaissance Festival", unless the player wants it all over their world.