Piketty's solution is a global wealth tax which would probably be ideal but with the political infeasibility I really think firming up the estate tax is hugely important. The generational transfer of wealth is a great time to tax it since the people getting it didn't earn it.

The maximum estate tax rate is 40% and that is much too low for a top end of a progressive tax, the people are already transferring $13 million untaxed so the 40% tax is much lower in reality.

Would be great to set extremely high tax brackets, like 75% of all transfers above $100 million and 90% of all transfers above $1 billion. If that disincentivizes you as a billionaire that's fine by me, you already won Capitalism.

And water is wet

No one needs more than $250 dollars million wealth, even if they had 10 children, they still live large and have plenty to leave to their children.