In the spirit that anything can be taxed, eg the number of windows as in early early days; I have few suggestions:

IQ:  The lower the IQ scores on a test, the lower your income tax (reportedly very popular in the T---p voters Camp).

Pen-s size:  The large pay more tax, Minnie wieners get a tax rebate (Cutt-off size yet to be determined)(Owwch)

Number of people hating you on Facebook:  This is for the HRC camp, the higher the number of hates, the larger the tax.
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Please, all in good fun, lt' s good to laugh at ourselves occasionally.