Suggestion: Universal Currency for Old Expac Vendors?

Wondering what everyone would think about adding a universal currency that can be obtained from almost any old world content, whether it be raids, dungeons, world quests, world bosses, etc., that can be used at new vendors to purchase old tier sets, old transmog gear, etc. So have a vendor for each expansion that sells that expansion's tier sets, and incentivize us to do old content weekly by rewarding this universal currency. So doing our BC or Cata raid lockouts can contribute to our Legion transmog hunting, or doing Legion World Quests can help us finish off our old Tier 6 or 7 sets. Old emissaries can have this currency added to them so we have a reason to go back and do our Legion and BFA world quests once Shadowlands comes out.P.S. While this is a whole different story, being able to queue for old raids and do them synced would be a good feature to add to this. Maybe make it so you can only get this currency if you're synced to the raid and not just steamrolling it. So how current bosses have AP tied to their defeat, we can make it so old bosses just reward this currency whenever you beat them.EDIT: No, you should not be able to buy it with gold. Outside of WoW tokens, which enable you to buy gold, you also should have to do the content to get the transmog, not stalk the auction house or do professions to make the gold to get them. They're raid sets. High-end PvE sets. You should not be able to get them outside of doing PvE. Small amounts currency from world quests, medium from dungeons, and larger amounts from raids and world bosses incentivize doing weekly resets. Gold would also not do anything to help nobody doing old content. A specific currency would have people going back every week and doing old dungeons and raids.EDIT 2: I've been discussing some concerns from some people in the comments and decided to make a more in-depth and though out post about this to put up later, and to submit as a suggestion to the devs. This was a small post with just a minimally thought out idea, but since it is clearly well received by many, I want to properly think through and put all of my thoughts into a single post later on. It'll likely combine a lot of the things being talked about in the comments, as well as some other thoughts that were not put on here to start.