I don't know if this is possible but why not let players keep the items they get and reset the levels. I find it to be a good solution. ( I mainly would like to keep my legendary gear from the iron banner that would effect the gameplay because it is cosmetic)
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They do... I'm pretty sure otherwise you wouldn't be getting the grimiours. Those add stat boosts as well in game during real game and the cards even say come back whennyounhave full game. I'm pretty sure the characters will even be able to use too. Although you might have to start the story over or something? Or maybe not at all? Idk I just know the grimiours keep the info of items you've gotten and stuff. And maybe keep some stuff in the vault and it won't go away! Haha I don't think it will through because your characters along with armors equipped and weapons are saved to bungie.. So idk of you will actually lose them