If I recall (I haven't read them in ages) The Mystery of Shadow Ranch involves horses, something either about a gold mine or a phantom horse or both, but that takes place out west.

Given the horse racing industry manages a great deal of money, prestige, and betting, I'm sure Nancy could be investigating things like....somebody did a bait-and-switch with a horse to be sold (sold a random no-name nag instead of the Pricey Horse it's supposed to be), someone has been sabotaging a stable's horses in or out of racing because grudge/betting debts/unfair shares in ownership-training-syndicate-rights, somebody's favorite horse has been stolen and/or is being held for ransom, somebody is mysteriously making a mint off betting (inside sources, sabotage, fraudulent betting)....

Alex, being a jockey as well as a stable owner, would certainly be able to facilitate her being able to get into private areas or speak with people who might otherwise brush her off (race officials, other owners or jockeys).

Any of that sound interesting at all? I do love mysteries but I'm not very good at plotting them out....