Half blankets, festive ones, trophy-style (I'm thinking of the half-blankets that racers sometimes wear, those trophy-style) and regulars... maybe with the option of wearing them with the saddles too.
We have regular halters... but why not rope halters? ๐
Did somebody mention bell boots? How about also shint boots, sliding boots, jumping boots and ankle boots.
Oh and then there's something like pretty bracelets to put on them like boots. ๐
Also, running and standing martingales and chest plates.
... speaking of jewelry for horses:
https://www.etsy.com/listing/1193845273/gold-or-silver-chest-piece-with?external=1&ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=jewelry+for+horses&ref=sr_gallery-1-3&frs=1&col=1&sts=1
https://www.etsy.com/listing/1239640407/faux-turquoise-teardrop-cascade?external=1&ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=jewelry+for+horses&ref=sr_gallery-1-4
This one is quite eye-catching:

And this one:

It'd even be neat to have breed-specific tack. I really don't care for the way this arabian looks (the following image), but it still serves the purpose. Plus, breed-specific tack would make drawing and implementing quite a bit quicker. With only one or a couple different breeds it can be put on, breed-specific tack might be quite useful, viable and quick to implementation.

And how about a full suit of armor?
๐ Okay... I think I'm out of current ideas... at least for now.