Guild Wars 2

I have two suggestions, who can be mixed in one.

When a player finish the first story episode, he win a special exotic weapon, this weapon cannot receive skins neither give skins, only can be equipped.

When the player finished all the achievements (including the story achievs) of each episode, he unlocks a new skin step of the exotic weapon.

When the player finishes all achievements from the season, he unlocks a token, this token can be traded in the NPC, plus the exotic weapon, for a new unique, exotic, Soulbound on Use, precursor weapon, attached to the Season. This weapon unlocks the finished weapon skin, like any other weapon, on salvage or use. This way, the player saves the precursor skin and use the weapon to craft their legendary..

If the player sells the precursor, he lose the skin, because he did not unlocked the skin.

This precursor is not the same as the old precursors. It is a new precursor which can be used in place of the old precursor. Like the old precursor, there is one precursor for each legendary.

Each season gives a specific precursor weapon, for a specific legendary. The season precursor is chosen, in a rotating way, by the devs and lore writers. IT’s a unique one-time skin, the only way to receive this skin is finishing all the achievements of the season.