Surely Gary is still an option as an overflow for ORD and MDW?
A better option than a new airport at Peotone?
If there is a relevant Spanish comparison, then is it not the ill fated Don Quixote airport? At least Gary is still in the Chicago area, and could be served by roads + potentially a rail connection if it warranted it.
Don Quixote (CQM) was a first, and presumably an only, in airport development - build a new airport and then connect it to the nearest metropolitan area by high speed rail - a journey of some 200km!
Of course, air + high speed rail works when you already have a megahub and are offering ADDITIONAL ways of getting there.
The other thing that killed CQM was that MAD opened T4, creating vacant space in the other, tattier terminals for FR, U2 and others to move in. ORD has also substantially developed its master plan lately.