I have an item concept kicking around in my noggin, its pulled directly from The Iron Druid Chronicles by Kevin Hearne (great series.) One of the characters (a druid) prefers to fight only with his fists, to that end he is gifted with a magical pair of Brass Knuckles that shape shift with him when he assumes an animal form. Now I am aware that you choose whether your equipment falls to the ground in your space, merges into your new form, or is worn by it. However, I am intrigued by the idea of an item that doesn't just merge with your animal form, but actually accentuates it. For example, in the books, the brass knuckles merge with the druids wild shape to enhance his attacks, as a bear they coat his claws, as a ram they flow to his horns, when he shifts to an eagle they transfer to the talons, even as a walrus the brass coats his tusks, you get the idea.
So, mechanics an balance; Ideally I think they would require attunement exclusively by druids, maybe +1 (depending on rarity), cause your unarmed attacks (in human form) to do 1d4 or 1d6, now this is great for someone wanting to fight with their hands in human form, but how can I structure it so that it can help a druid in wild shape, especially moon druids who have access to high CR beasts who already do decent damage?
If you don't have the book, it's basically a +1 to natural weapons (just like a +1 sword). You can use it as a base and then change the flavor of how it looks.
I've always liked the idea of gauntlets or bracers that change shape along with the form of the animal shape the wielder takes like you describe, until I get to a beast that's primary attack is a bite. Then suddenly I think of a dire wolf with diamond studded gold teeth, and I'm suddenly less interested lol.