I think it would be amazing if we could use mission badges to obtain rewards from previous mission pass seasons. Currently, there are a limited about of things you can even use mission badges for, and the shop seems to never change out for new items. Being able to spend badges on previous rewards might take away from the "prestige" of having obtained it when it was released. However, it would also incentivize buying a current mission pass even if there isn't a reward in the lineup that you want.