Any real changes require the political support that the very same monies and interests are guaranteed to stymie. But anyway:
* Close obvious tax havens by shuttering all trade and financial flows until the white flag comes up, and only then open trade progressively as local supervisory institutions and banking practices pass muster.
* Close the other tax havens (looking at you, Netherlands, USA, UK, Singapore) by, who knows, agreeing to stop the nonsense.
* Place a limit on ownership nesting, such that only one level of holding company is allowed in any corporate structure, max. Gets too big? Spin off divisions into new and still similarly flat ownership structures.
* The top investor in any private company, large or small, holding or not, must be a physical person with a known place of business, and physical residency in one of the top two, maybe three countries in terms of company sales.
But now, surprise:
* Eliminate all corporate and business income taxes.
* Set the retained earnings payout ratio to the rate needed to be tax revenue neutral (can't remember since I did the calc a long, long time ago). Was something like 40%.
* Allow some leeway for retaining more earnings to accumulate investment capital through certain forms of provisioning.
* Personal income taxes are the same for earned and unearned income.
Finally,
* Introduce a flatter, but not flat tax rate, now that the tax base is a great deal larger, and fairer.
* Everyone wins in the end.
* The flat corporate structure remains, as it is key for retaining proper legal liability and accountability.