We all know that Doom is at the moment the only T9 with Two Seater, which is fine, atleast for now!
BUT! With the Snow Region coming out, there would be an option to give Doom finally a true different unique Skill (two seater isn't unique since a lot of horses can learn it).
For the Snow Region you could add a Skill for Doom which gives you immunity against the Debuff there, similar to the Unicorn in the Desert.
On this way, every T9 would have their full unique Skill Sets and Two Seater could be learned by all T9's, or maybe even just T10's if you wish so.
Since a lot of people love to take their friends on a ride but don't like the Doom Horse or prefering another one this would be a great option to change in my Opinion.