Elizabeth Warren

The 16th amendment is very specific. It applies only to income tax. All other taxes must be on a per capita basis.

Her idea could also run afoul of the prohibition on a "bill of attainder," which prohibits a punishment of any limited class or group. That requires considering a tax as a punishment, but when applied selectively such an interpretation is far from outlandish.

There is also the ban on "ex post facto" laws, imposing conditions after the the action occurs. The tax was not in place when the wealth was accumulated, which makes such a tax "after the fact." The person might have conducted financial affairs differently had he known that accumulated wealth would be subject to tax.