I think you need some "givens" before you start planning mass migrations.
Things like:
- I think we can limit the mass migration from where ever to the USA??
- Mexico: the boarder is a shallow river and/or patch of sand that is traversable with limited resources
- India: is semi-isolated with a vast water boundary on three sides, the forth is 40-60 % mountains that are nearly impassable.
- Africa: again surrounded by water and only on the North side could you traverse with nominal transportation and resources.
- South America, with it's water boundaries also has a natural choke point, The Panama Canal.
- Other obstacles of continental proportions limit the mass migration of peoples in any direction.
So while there may be masses of individuals that want to migrate, their transportation is limited because of nature. And then the UN,, who may volunteer to transport the mass to us as a USA World tax..
The exposure to the US can be netted to Mexico and Canada. With some cleaver PR work, we may be able to pipe-line the masses directly to Canada???
This might be a bit silly,, but the key is transportation, with-out which nothing moves.
Unless some entity wants to volunteer to transport bazillions of bodies,, they will stay were they are.