Id be curious the surface area of those million population centers? Lots of the US is very spread out even for “cities” only the old cities on the East Coast have significant density.
Zoning to prohibit people from living on 99% of the land so that the population is dense enough to fully benefit from public transit? Lots of states don’t even have a true city at all. Should those be zoned as national parks?
Edit: I think I missed the point. You’re talking about just city zoning. Still, not a magical fix and would likely require moving lots of people and demolishing/building buildings and infrastructure. However, it’s more realistic than zoming everyone out of the midwest.