You get your basic income check for X dollars a month. If you work a job, run a business, whatever, in addition to that, you get taxed Y% on your earnings, excluding the basic income amount. No deductions, no write-offs, no tax-shelters. If you wanted to have a progressive sliding scale tax rate, fine, although that makes it more complicated than a flat tax.
HR Block and Intuit would scream, but if you could do your taxes with just your W-2 and a four-function calculator, how many billions of dollars could we avoid wasting and invest in a more productive sector of the economy?