Well, spellcaster feats are a different vein than weapon users. Some potentials:
If you plan on using a lot of ranged touch spells, you may be interested in point blank shot and precise shot. The -4 for shooting into melee applies to ranged touch spells.
If you plan on using a lot of spells that are dependent on the enemy saving throw, then you'll need very high casting stat, spell focus, etc.
If you plan on throwing spells against difficult foes, like demons, you'll need spell penetration.
And if you want to cast spells with any reliability whatsoever in dangerous situations, you'll need combat casting and possibly the trait that does the same thing, for a total of +6.
Those are just some of the starters. For a hybrid like FJ, perhaps precise shot path. Then he can benefit with ranged physical attacks and touch spells. He's kinda screwed on 'save or x' spells due to having only moderate INT (for a caster) and access to lower level spells (because of multiclassing), so he's not a good candidate for that path. Combat casting may be worth it, overall.
The only downside of working on your combat riding is you're going to get into the position where a mount tough enough to use regularly will no longer be medium or less in size, and you're going to spend more and more time off the mount, much like the horse riders already do. If I had a dollar for every cavalier in Pathfinder Society that wanted to ride his horse into a dungeon, I'd be, like, $3 or $4 richer. (But really, should anyone even ask?)
As a counter argument, one could say putting a trait into riding isn't too great an outlay.
