Would it be feasible to create a sovereign wealth fund coupled with an unrealized gains tax in the US?

Politics aside, what would be the downside to such a plan? One of the oft-cited arguments against a wealth tax is that it’s too difficult to tax unrealized gains. So why not create an “unrealized tax”? A certain percentage of holdings from individuals or entities that have total holdings over a certain threshold get transferred to a sovereign wealth fund. The fund could then be structured with an automatic liquidation mechanism similar to a 10b5 1 plan as to not cause major market disruption.