Several weapons and sentinels have numerous variants, between normal, prime, wraith, vandal, prisma, kuva, tenet, etc. I think it would be good if once you have fully mastered one weapon's variant, that you should be able to use that variant as a skin on any other variant of the same weapon. For example, having mastered Prisma Shade, its appearance should also be unlocked as a skin for Shade Prime. I know that prime variants can already use the non-prime appearance as a skin, but not any other variants. This would be a way to allow greater customisation for those who prefer gameplay unlockable cosmetics to purchased ones.