i think this would be an awesome idea theres already mods for clown-ish clothing and animal taming the rest i assume would just be making the NPC's and set there travel route
this would be awesome skyrim definetly is lacking in minigame type mods i think that if someone does make a Horse Racing mod they should do it kinda like the racing in the legend of zelda oot and Majoras Mask keep it simple but fun