I think I am fine with taking an approach to that that is just brutally tilted against large values and brutally tilted in favor of transparency. So like, you can't do anything with your ownership share in that privately-held company --- can't use it as collateral for a loan, can't present it to investors to get more funding, can't trade it, can't sell it, can't in any way derive any benefit from it --- without committing yourself to a valuation and paying a tax on the increase in the valuation since the last such assessment. Also you can have a "sound dues"-like system where committing yourself to such a valuation also gives the government the right to immediately compel you to sell them the asset at your valuation. Any inaccuracies or procedural missteps in these calculations will incur minimal penalties until the amounts in question rise above a threshold (maybe like $50 million), at which point attempts to conceal or misrepresent the value of an asset is punishable by an increasing share of the asset, scaling up eventually to total forfeiture. All in all it should be excruciatingly painful to accumulate anything approaching the large wealth holdings we have today. Most of the large privately-held companies simply should not exist with the opaque valuations they have today. Either open the books completely, or lose everything.