I apologize if this has been suggested by any of the aforementioned larger genetics threads, but perhaps it would be an interesting idea if marking heredity weren't so binary? I'm particularly thinking about special markings, and I was inspired by the new "bicolor" eye colors.

My first thought is for newer, rarer markings to "fail to" related, older markings.
E. g. Vitiligo 4 > Vit 1, 2, 3
And a "vit 5," if it ever exists, could fail to all 4 previous vitiligos.

This could create a nice silver lining for if you failed to breed a marking, but the upshot is that perhaps passing on the actual marking itself could be more rare – say the chance is 50% now, maybe it would go down to 25%.

In the future, perhaps special markings could also come in sets from more common/easily inherited to rare.