Honestly if they burned it sunlight it would make them a very interesting horse alternative. They are already tamed, normally have more health, are more abundant, and finally, they are faster and jump higher. Since they are usually all around better, making them only viable for half the day is an awesome nerf that I'd love to see.
Also, some general all around horse buffs might be cool. Right now, horses are basically unused as they die too easy, are hard to find and not much better than normal travel (even if you disagree with this read on). There could be some fun stuff done to make horses much more interesting. Imagine if you could level up your horse by riding it often and training it. Now you can make horses better by training them and perhaps we can even find ways to mark a horse as soulbound. This would mean the horse will always re-spawn in the Stables of your town after a re-spawn counter.
The Stables could be a new industry based of a town farming level that simply allows horses to re-spawn after a count down. The higher the level of farming the shorter the re-spawn counter. Finally, You can have an input chest that uses golden apples or hay bales as input for re-spawning horses. This way the re-spawn isn't simply free and actually is worth something. And finally to keep inline with most other industries, an output chest could display the names of the horse that will re-spawn next. The items in the output chest would be unmovable and only to show which re-spawn will happen next.