A "flat rate" can also be tiered. In this case I'd take it to mean a flat rate on all money earned during a time period. Whether income or capital gains, dividends, assets or whatever, you are still giving up x% of whatever you earned that year.

So even when tiered, a person earning under some amount in total might get a lower tax rate than a person earning a higher amount but it is still flat.

The "flat tax" would be taxed on all jurisdiction. So call it a "universal flat tax". That way, there is no persuasion to register in a foreign country and it would dissuade multi geography registrations as you would just be taxed in multiples by each territory.

To avoid double taxation, perhaps, a company can register in multiple countries but since the tax is the same everywhere, the portion of the x% taxed would be proportionally divided to the territories.

This assumes of course that the whole world agrees on one tax policy and tax havens are eliminated. :)