Please i know it may sound ambitious but is there a way of creating horse breeding ranches? Be able to buy a plot of land build a ranch? Hiring workers being an option to help out around the place. Buy horses off other stables or tame wild beasts, bring them to the ranch and place two of the chosen horses in a breeding pen with X amount of food, how ever many weeks go by in-game and naturally have to be cleaning out the pens brushing, exercise etc then voilà a baby nag is born for the player to rear.
Even just raising work horses to turn a small profit at a livestock market would be fun.
On the back of that Horse racing could be something real fun to look into. Breeding thoroughbreds for racing and then competing against other race breeders. Being able to place bets on races would have to follow (something the rich folk of Saint Denis would lap up). Possibly rigging races, leaving good or bad betting tips and those actions having consequences.