Equal taxes in each megaregion is something to take into consideration. Take into account how the NorthEast Region works. Let's say New York City and New Jersey, both part of this huge metropolis. Because New Jersey has cheaper property and state taxes than the city, residents of New Jersey pay less but enjoy many of the benefits and public spending of New York when they come into the city and work. A megaregion could benefit if they could effectively tax New Jersey residents.

Where I'm from originally in Pittsburgh there is the same problem. The city of Pittsburgh has a transportation system that they pay most of the bill for, but they can only collect taxes from residents of the city, which is pretty small. If they could tax everyone in the county and everyone effectively in the economic region then they could more equitably pay for the bill for the common resources that everyone enjoys. In Allegheny county, you have people who decide to live just at the border of the county in a place called Peters Township. It's close enough to the city that you can commute to Pittsburgh, but you do not pay Allegheny taxes. Kind of unfair.

I disagree with your 3rd point.