Maybe you could consider adding a triat for twins or multiple births. It could be passable genetically but of coarse fairly rare. But if it were something that we could breed for it would be a really cool thing. if it were something that only occurred in more common breeds until bred it might be a way to give them a bit more of an edge and incentive to breed a common starter coat to a more expensive coat and keep their value. Another thought would be that older ones or ones with a higher regard and age might be made more valuable if they had a higher chance of multiple births.
I'm hoping that you've added in ways, perhaps not necessarily this but its a thought, that will allow people to have animals that are desirable other than running out and popping 100k for the latest and newest coat/eye or simply hoping to hit the random genetic lottery and keep a bit more of an even playing field.