Until there's a wealth floor -- until everyone is fed, sheltered and has access to education, there needs to be a wealth cap. We, as a collective humanity, should not allow anyone to have more than, say, $20 million, until the basic needs of others have been met.
That would incent people to spend on consumption when their wealth approached the cap, while not actually paying any of the tax. Thus more of the world's productive capabilities would be redirected towards rich people consuming luxuries that they otherwise wouldn't have bothered with.
Not saying I disagree but to play devil's advocate here - wouldn't them spending on luxury items still benefit society overall rather than them keeping it locked up somewhere? Working class people are the one's making and selling the luxury items and once injected into the economy the money they spent on said items would be taxed further and spread around as others spent it.