I personally love this idea. Great job on all the mechanics of the commands and sequence of actions, they are all decent and amazing, somewhat cosmetic, features that would add to an avatar's experience.
However, you can not save skins of a player sadly, only the current one can be applied to NPC's.
Once a player changes its skin, the past one can no longer be retrieved via coding API's such as Spigot, or Bukkit.
However I do love this idea and I hope it is taken into consideration!
Possibly, and just a concept, if the player chooses to enact this command or feature on a solstice, the avatar can boost one element (Which ever main element the past avatar had) by double or a third of the normal avatar state configuration options?
For a visual reference:
Avatar Roku assisting young Avatar Aang at Roku's Temple against the FireNation during the winter solstice.
~Apple<3