Piketty's 2% tax floor on wealth (for those with over $10M/$100M in wealth) is an excellent way to fix this.

Basically the idea is to just accept the fact that the super wealthy will find the loopholes, so instead remove the loopholes. Just flatly put a minimum tax all wealth over $XX million at 2%.

One nice aspect of the policy is that it's a tax minimum: so people with high incomes who are already paying that much, regardless of how wealthy they are, don't need to pay the tax.

Another nice aspect is it's relatively easy to implement. There aren't that many people with $XX millions in wealth.