One practical approach is to have a high income tax on large incomes with a tax break for income which is invested rather than spent. There is something a little like that in the UK where you can offset 50% of your investment in startup against your income tax bill. I'm not sure how economically efficient that will prove - a lot of the startups go belly up. You'd probably want the income tax a bit higher than the present 40-45% to keep Piketty happy.