a tax on unrealized capital gains of any kind is idiotic; literally taxing imaginary wealth. The "unintended consequences" of such a law would be a disaster, in two ways:
1. as intended, by focusing on 'billionaires', the tax will force mass liquidation events on young volatile companies, wiping out the very wealth the law sought to tax (and not just the billionaires' wealth, either)
2. as not intended, assuming the notion of taxing unrealized capital gains (i.e. theoretical gains on paper, not actual income) survives logical and legal challenges, when the "only on billionaires" part gets dropped and "also on your house's market value" comes into play.
The first income tax was a tiny percentage on millionaires. It didn't stay that way, neither would this.