@Travis Gibson AFAIK no one does plastic plains Indians. (Once apon a time, in a previous millenia, Airfix did 20mm ones  - a lot of which ended up being converted to Ancient Brit or Hyborean cavalry under old school ancient rules ).
Boxes of 28mm US cavalry & plains Indians inspired by those old figures may be viable. As the Airfix originals, have both mounted & unmounted figures in the box.
I'd suggest using the stock horse sprue for both, say 6 beasts for both, have 2-3 split figures (as with the Rifles), lower torso mounted and standing plus saddles and as many extra standing figs as would fit on a sprue with 3-2 sprues per box. Done cleverly, both boxes would become a goldmine for kitbashing (as were the airfix originals). Also make the US cav scale compatible with the Perry ACW range.
Even consider a 28mm Airfix Wagon Train inspired plastic Box ?
Forts, civilians, chiefs, scouts and misguided senior officers are all potential for 3d printing.
 Airfix Wild West
 Donald Featherstone