Finding a meaningful currency would be the most difficult thing. If you make it gold then what do you use more gold for when you have 100+ billion? Unless someone has enough keystones to never need to run a yellow rift then its really hard to not have more gold than what you need. The nature of farming bounties, keystones, DBs, exp pools means you are most likely going to be encountering gold, especially with classes that pair up goldwrap + avarice + hoarder gem into their farm sets
If you made the currency DBs, which is the only somewhat limiting currency then everyone just runs around in sages sets on yellow rifts 24/7 and the bots scrape all the good gear when people figure out they could post things for 10k or 20k DBs and the botters would buy it
Sadly it might just be too late in the game to implement it. If they did something special with Season 12 and made anything dropped during Seasons tradeable that might be kinda fun just to mix things up. They could get a free 3-4 month test run that way and may even spark some more seasonal interest beyond just the first few weeks