A wealth tax. All assets and cash need to be properly valued and then that value taxed. I agree with the sibling comment and also with past discussions where a wealth tax is the only non-regressive (not-keep the poor down) tax code.
One benefit of markets is price-discovery. But if you're not participating in a market, determining price is akin to shaking a magic 8 ball. It assumes a Just Price, which is a heuristic and a fiction.
Also, quality of living is determined by income, not wealth. Fresh retirees are generally wealthier than other age-groups because they've saved for retirement. But that doesn't mean they consume more. Also, consider the citizens of the Netherlands. They enjoy a comfortable existence while servicing an enormous amount of debt. If wealth were the primary determinant of quality of life, you'd think the Netherlands were as as destitute as Somalia.
The retiree example is intended to demonstrate that a wealth tax is inherently perverse, regardless of who is targeted. Savings are just deferred expenditure and debt is just expedited expenditure. When you tax wealth, you expedite consumption.
Another way of thinking about this: A wealth tax is like inflation, except assets are also devalued alongside your savings. Which means the wisest strategy is to consume now, save nothing, invest nothing.
50 dollars tomorrow may be more useful to you than 100 dollars today if today you'd only buy useless crap for it but tomorrow there is a brilliant investment opportunity.
Things still get complicated there. What is the value of a car? I'm not planning on selling mine, so I have no idea. Do I just Blue Book it? If I key the shit out of it every Christmas and then have it repainted in January, can I knock 40% of the value off? It might be cheaper to just have it repainted.
What's the value of a painting? You could use the price I bought it for, but what if we just swap paintings instead? You know, I give you this fancy painting, you give me some Goodwill shit and you pay me a $100k "finders fee".
What's a yacht worth? What are super fancy houses worth? One of Mike Tyson's old mansions is still abandoned, so I assume it was worth nothing since they couldn't sell it.
Properly valuing things is hard. Wealth taxes are also regressive, in a way, because poor people are much less likely to have access to financial instruments that will return greater than the wealth tax rate. The poor and the middle class are far more likely to have their money in functional investments (homes, cars, etc) than they are to have them in something that generates profit.
So you'd still have to just not assess the tax on assets under the value of a reasonable home and car, at which point we're basically just back to what we have now, which is carving out a loophole for the poor.
I'm not sure that would work as simply as that. Imagine buying a volatile stock, say, TSLA at $100. This then skyrockets to $1000. You're now taxed on your wealth at that price point.
Then it immediately crashes down to $120.
All this while, you're just holding the stock but paid taxes on a non existent value.
I highly recommend that people look into Islam's Zakat laws. Zakat is a form of "tax" if you will, but is much superior and has been proven to work historically. Money that has been sitting in your account unused for a year is "taxed" at 2.5%. Things like livestock and produce have their own rates.