I agree with the goal, but I think a better approach would be to tax them on investments used to get loans. They DID realize the new value of that investment by convincing a bank it was sufficient to secure a loan. So make them pay the full taxes on it as if they'd sold it at the time the loan was approved, and only make them pay taxes on the difference from there if they actually sell it. Get a loan based on $500m investments, pay taxes on any profits accrued by those investments at the $500m value, and if they sell later at $600m, only pay on the new $100m, or if they sell at $400m, a tax deferment as appropriate for a $100m loss.