This is not the end all solution but it at least makes everything far, far simpler.
Most corporate tax avoidance strategies take advantage of laws intended to prevent different countries from taxing the same dollar of profit, and those rules will still exist in any reasonable tax structure.
The real problem comes when these deals disadvantage local businesses competing with the big foreign conglomerates. But often these deals are really only necessary because the government has set up ridiculously high trade barriers, so the only way to get investment is with ludicrously generous deals. Brazil seems to be a classic example of this. The entire economic system is tangled mess of massive market distortions.
You're mixing up terms here. Tax avoidance is minimizing your tax bill within the law. Tax evasion is what you do when you pay less tax than your legal obligation.
However, the direct and indirect pressure large companies exert on governments to promote/create these loopholes is absurd. I recently went to a latin american country which is promoting foreign investment. Guess what tools they are using to promote such FDI? Correct, tax credits, tax agreements with tax heavens, zero withholding taxes on dividends, etc. I won't go into more detail, but I've seen obscene (but legal) tax structures were little-to-no corporate taxes nor capital gains taxes have been paid in +billion euro transactions.
IMHO, this is unfair to everyday citizens. Citizens end up taking the burden of supporting the govt budget, at the expense of higher tax rates.
Unfortunately in the EU, there is very little interest from countries like The Netherlands, Ireland, and Luxembourg in implementing a common fiscal policy, with no loopholes, as this would considerably damage their "competitive" economies.