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!

Has any1 tried creating a unicorn breed? Is it possible?

..Be fun to have sumthn similar to the warlander in perlino or white but i dont know how you would get a horn. Willow of Emberwilde

I like the Elvish Warhorses - it's a custom crossbreed campaign and most of the breed are very colorful with fun patterns :) I've not bred mine for a little bit because I switched back to focusing on my Irish Draughts, but I really love the ones I have! 

One I like is the Spanish Jennet crossbreed campaign! It's 25% Appaloosa and 75% Paso Fino. We're specifically trying to add spotted/speckled/mottled patterns to the breed, which is both more of a pain and more rewarding than I was expecting!

And not to self promote, but I'm trying to start the Medieval Palfrey as a crossbreed campaign, which is known for their coats. So I'm trying to add "shining" effects like cream/grey/pearl/birdwatcher spots/etc. Basically a unicorn without the horn, lol.

Ooh, the elvish warhorse! I've been wanting to jump in, but it's a tough challenge. Some of them are so pretty though!

Hi Dovey! owner of the Elvish Warhorse campaign here- if you want to get started I have a bunch I was using for a giveaway that I'm no longer doing, so if you're keen on getting into them send me a message and I can give them all to you :) 

Just make sure you join the club first, so you don't have to register them all again once they're sent over.

That goes for anyone else interested too, I can send you some to start out or breed a batch of foals for you