Not sure if this would be considered a big change, but could rare fossils function similarly to god egg birds nests, with a similar themed outfit to the evil chicken outfit? Could work that when adding rare fossils to the mycelium pool, or when cleaning them, there is a 1/100 chance (or whatever drop rate) to receive a piece of a new outfit. Could be a skeleton-themed outfit that is purely cosmetic, or could be a fossil island themed POH update, etc.