Coat Colors
I'd honestly love to see a horse with the pearl gene
Mosaic/Horses with chimerism could be really fun since theres a tone of diffrent combos that could be done
I don't know if this counts but zebra and zorses could be fun as well

Gameplay:
I agree with maybe some cosmetic things such as barding/saddles/blankets/accessories for our horses
They could be earned like other collectables/companions and could help individualized each pony if you happened to have multiple of the same colored pony
could be listed in the stall/pasture like [pony image] [dressed up pony image] to differentiate them from LE that come with accessories
Could also add a dress up competition with this feature. that could be really fun around october.

Aside from that maybe Ribbons as a collectible won from competition?
It would also be neat to have some more competitions that work off more than one stat as well.

sorry if this was a bit rambly. I just kinda wrote down ideas as I had them.

Oh I love sacredscales' ideas! The tack is a great way to add many many new collectives and it's an opportunity for further dressing events etc. And we could use the tack in our roleplays.

I love the livestock idea with the possibility of selling our production! I was also thinking about growing plants - maybe crops but also decoration plants to collect.