Having a Shark/Turtle OC, this is a brilliant idea that would certainly allow me to brig Shurtle to his true potential. As, above his shell, he has a fin, and that would be incredibly sweet to be able to have it. Awesome idea!
Although as others have said, it's likely that you'll have to make your own items and implement them into the game as an item pack yourself, or even commission a FeralHeart item-maker to do something for you.