I know people have suggested polydactyly as a mutation in the past, and it may even be on Xylax's planned-to-implement list. However, as someone who has had multiple polydactyl cats, I wanted to talk specifically about different variations and how they might provide boosts or penalties to stats accordingly!

- Front paws only (two extra digits each - "thumbs" or "mittens" version - preaxial polydactyly): Possible small stat boost (+5 agility?) because the lion can grip small objects a bit better, using its extra digits almost like opposable thumbs; otherwise no in-game effect. The cats that I've had like this had a bit of reduced flexibility in their extra toes but no trouble curling them inwards, they just couldn't flex back as much.

- Front and back feet (two extra digits each - "snowshoes" or "patty-foot" version - mesoaxial or postaxial polydactyly): Reduced agility (-5) because of big, clumsy feet, possible messages about the lion having trouble with climbing or manipulating small items. One of my cats had this type of polydactyly and seemed to have a reduced ability to grasp well, making me think the connective tissues/tendons in his paws hadn't developed properly, though it didn't seem to cause him any discomfort. The large feet might actually make walking on sand easier!

- Front and back feet, mixed type (thumb-type front paws and snowshoe-type back paws): No in-game effect (stat boost and penalty cancel each other out).

- Optional invisible mechanic: Starting at 10 years old, a polydactyl lion might develop arthritis in its paws because of malformed bones in its extra digits. This would provide in-game messages and could even result in possible early death ("LION NAME died last night when s/he lost footing climbing up a steep slope and fell," or "LION NAME went out hunting and was killed when his/her grip on the prey slipped and it trampled him/her").

Hypothetically, since we don't fully understand the genetics behind polydactyly in cats, I would think that a parent with one version ought to be able to pass its version and also the others to offspring. We do know that polydactyly is a dominant trait, so only one parent would need to have it.

Here is a research article on polydactyly in Maine Coon cats with some pictures and x-rays of different presentations!

https://www.researchgate.net/publication/293807417_Clinical_characterisation_of_polydactyly_in_Maine_Coon_cats