Many questions remain to be addressed; some have thoughts about their answers.
Should the cap be on net wealth or on total assets?
A cap based on net wealth may encourage indebtedness, but the effect could be small.
A cap based on total assets may complicate pre-ordering schemes, debt-based investment and financial leverage -- but only if not done through limited liability companies, which would not be capped and which would not increase in net value through these; how common is it for sole trader companies to have millions in assets?
What should be done with the debts of those capped, particularly if the cap is on total assets and particularly if the debts exceed the value of the cap (a billionaire can handle a debt of hundreds of millions)? Should (some of) the debt be cancelled with seized assets? The debt itself could have been seized by the state as assets of another capped person.
How should a government implement a wealth cap, to reduce economic turbulence and ensure its effectiveness? What difficulties would be expected? How would they be overcome?
Could have worldwide tax/cap jurisdiction for citizens (as used by the USA -- and Eritrea!), to reduce (the effect of) capital flight. This is likely the best option.
Could have capital controls, perhaps specialized ones for large fortunes.
Could apply the cap retrospectively -- ignoring the recent movements of people and capital. This is likely the worst option.
Could/should anything be done about non-resident/domiciled non-citizens with large interests in the country?
Capital controls and/or restrictions on foreign ownership may have bad side-effects.
Opaque arrangements of shell companies may complicate capping only holdings (houses, shares of local companies etc.) within the country.
Perhaps foreigners are relatively insignificant, politically.
What would be the economic downsides of a wealth cap and what could be done to mitigate them, both after enactment and as part of the wealth cap itself?
Probably turbulence, plus maybe a small net loss in GDP from some unmotivated or sulky (former) plutocrats and 1-percenters.
Some of the seized wealth may be used as economic stimulus.
What economic upsides would there be from a wealth cap?
Less income inequality as incentives for super-high pay decreased, therefore either higher dividends for shareholders or higher pay for lower level workers etc..
Or sooner retirement for the highly paid, giving more people the chance to take such well-paid jobs.
Directly, less inequality of income from capital (dividends, rents, interest, capital gains etc.).
Decreased power of super-rich would result in economic policy that better favours poor and middling people.
Other than Sovereign Wealth Funds, what could be done with the seized wealth?
Selling it off would risk transferring it to foreigners at too low a price, as its sheer size would distort the market.
Redistribution is likely to be messy and difficult, both morally and practically. It may also cause inflation (at least for mundane goods; other prices may already have fallen from decreased demand from the rich).
It seems that over the medium term the state would need to keep the wealth, as a Sovereign Wealth Fund dedicated to a particular programme. The obvious choices are healthcare and pensions, as they would likely be the most popular and politically unassailable options.
What proportion of a house price index and a general inflation index should be used to determine how the wealth cap level would vary over time? The cap should always be high enough to not deprive many of financial independence. Financial independence is the difference between a haircut and a scalping.