Comment by DoveBrown

I'd personally prefer if we ameliorated your paid tax so that the rate applied over your entire lifetime earnings. That way you could use the same progressive rate but windfall years aren't a special case and lean years are an example of failing to optimise your taxes. You could even have the government refund tax if the windfall event was a once off.

Extra bonus you can get rid of many loophole taxes (like Capital Gains and Inheritance taxes) since these feel unfair to take on a single year basis.