Well, I would say that the IRC would have to be rebuilt in a comprehensible way.
Simply defining "taxable income" in a way that doesn't contain any unintended consequences and doesn't contain any loopholes is, well, impossible. Granted, you could simplify some little things, but for the most part the IRC isn't even the complicated part - it's the treasury regulations, the revenue rulings, and the court cases which interpret the IRC where things get most complicated. The IRC could simply say "There is hereby imposed on the income of every individual a tax of 25%" and you'd still have the vast majority of the complications deriving from the interpretation of that rule.
As an aside, the so called "Fair Tax" suffers from the same problems as the income tax, because it doesn't tax items purchased for use in business.