I've made a couple of suggestions in the past. One of them was really dumb. One part of it was actually something I want to have in Niche. A second pattern slot for white underbelly. This gene would have multiple variations, the most common being "no bicolor" or something like that. Maybe additionally a size modifier gene? You could get different variations of it from wanderers and you would unlock the mutations for those as you invite them in. The white underbelly would cover the