This sounds terrible. The whole idea is that you don't realize gains unless you have to. That is, don't sell your long investments! If gains are unrealized, how do you even pay? You must sell your assets to feed hugry monster called government.
Fiat currency is a tool created by governments to accomplish goals. Massive income inequality generally conflicts with most liberal democracy's goals. Taking loans against unrealized gains lets people much richer than me pay a much lower effective tax rate.
Why not invent a new tax to address this?