An upper income tax and wealth tax bracket defined annually by a multiple of mean GDP per capita, and set at 100%. This rich, and no richer.
If you want to get beyond nx wealthier than today's average citizen, you're going to have to make tomorrow's average citizen richer. Want your slice to be bigger? Enlarge the cake.
N income and n wealth could be really big, it could be possible for individuals to be filthy rich and still have this work, IMO.