Suggestion: create "proxy individuals"—paper people that legally own all of a foreign national's U.S. assets, including their bank accounts and their corporate interests. Then tax those proxies at all the points you'd tax regular citizens, including capital-gains time. Disallow foreign capital ownership without proper "taxability insurance backing"—i.e. seizable assets—existing in the proxy bank accounts.

That's a reasonable point, but it turns out nonresident aliens are subject to a flat 30% tax on dividends (could be lower depending on various treaties).