Probably not a popular breed but Hispano Arabes have very very few people breeding them. I have a horse I haven't bred in multiple in game years because there's just nothing available and it makes it really hard to breed to better the breed in game. It's an Andalusian crossed with Arabian.

Quarter Paints have a lot of variations to them (Quarter Horse x American Paint)
American Warmbloods also have a lot of variations (Thoroughbred x Clydesdale)
American Aztecas (Andalusian x American Paint) or Aztecas (Andalusian x Quarter Horse) may also be what you're looking for.

Drum Horses are Irish Cob x Clydesdale, so you have a lot of color variations esp due to the Cob.

If you're wanting Crossbreed Campaigns instead of the game crossbreeds, you can also look into Warbreds (TB x Warlander), Celtic Cobs (Irish Cob x Welsh Cob), Walkaloosa (Appaloosa x TWH), Appalachian Mountain Horse (Akhal Teke x ASB), Georgian Grande (ASB x Clydesdale) and Moriesian (Morgan x Friesian).  Those clubs are all fairly active.

If you go to the Clubs page, and the top right corner go to Crossbreed Campaigns you can look at what's out there. Just be aware if the horse registration is set to private the club owner has to be active in the game and approve registrations. I've stopped breeding for a couple of clubs that aren't active for that reason.  :)

Have fun!