I am really excited to see that you are doing another version of this game! First and foremost, you really need to improve the horse art. A key appeal to any horse game for the majority of horse-crazy players is having a visually appealing horse to look at! While I have long wanted a good stable management sim, the really fugly ponies in Stable Tycoon 2 made me put the game down and walk away from it at times.
Second — I would love seeing more business management sim involved. That seems like a really under served horse game market niche across every platform . There was a very fugly art game called Welcome to Trotterville a few years back which began to get into the very interesting business sim aspects, but again the art quality was terrible and the programming was massively buggy.