This is very unlikely, but if people will throw out ideas for what would make great, substantial plots and characters for a sequel or, goodness granting, a trilogy.
My overall idea is:
Spirit, Rain and the herd are reunited, but the theme of humans taking over the West still hangs over both the horses and Native Americans. Depending on what year(s) this is supposed take place, General Custer could come in as another antogonist along with Cromwell.
Possibly, Spirit and Rain have a kid. Perhaps it is better to focus on just one kid than two. But Spirit and Rain would be the central characters still. Maybe there are other herds and tribes that the Cimarron can unite with to make a final stand against the Army/Railroad Company, etc. A more epic feel, while retaining the 'spirit' of what makes the setting and characters so special.