A flat tax is what a flat world needs!
What I would suggest is to go instead for a real worldwide tax transparency by making all countries sign up on an easy to understand world wide flat tax. This would help to remove the incentives to geographically arbitrate taxes and that keeps so many accountants, lawyers and financial institutions in the business "legal and intelligent tax avoidance"; and that keeps so many governments from not knowing whether they are giving true and needed tax incentives to attract investments or just being taken for a ride.
In all, a flat tax is precisely what a flat world needs; and by the way, just in case, a flat tax can be construed as a progressive tax too.