So, that is one specific way to tax the wealthy and isn't even one that I am thinking of.

The biggest loopholes to close are:

1. Caps on things like medicare that are a percentage of income with a cap. Just remove the cap and make it a percentage of any amount of income.
2. Raise the taxes on capital gains to align with income tax instead of being a tax break and apply percentage of income taxes like medicare to them. Income is income, and unless capital gains is taxed higher than regular income it should just be income because it is treated like income.
3. Lower the caps on tax breaks for things like home interest. Currently the rich can buy multiple homes at interest rates lower than the rest of the population and then they get both income from selling it later and it makes them money just by being able to buy whatever. Tax that process more when the house is worth an excessive amount.
4. When they get a loan against stocks and other assets it should be taxed as income if it is above a certain threshold, say the median household income. So if the median household income is 100k in some area, then them getting a cash loan should be taxed for every dollar over 100k as income. This closes the loop where they don't need 'income' from jobs because they get assets that appreciate instead and avoid income tax while never paying taxes on those assets until they sell them.
5. Don't let them lower their taxes because they 'lost money' in one year. Regular people don't get to do that, don't let them either.

There is even more that goes into the process of them getting massively rich while avoiding taxes by business and asset appreciation that doesn't count as income, which appreciates faster when they are underpaying their staff. The whole problem is how fast having money lets you get more money without paying taxes.

And don't get me started on tax exemptions for businesses that are never a net benefit for the community they are in. That is a whole race to the bottom.