What I would like to see is making the horse a creature that has its own AI and such. To make it move forward, make it go to a xmarker that is dynamically placed in front of it. That way, the horse will follow the navmesh, and will act more like a real creature instead of a biological motorcycle. I'm not sure if it would stop them from jumping over cliffs, but if it did, it would add a lot of immersion.