In 1985, a proposal was floated for twin stadiums to be built in the North Loop area, one for the White Sox and one for the Bears. As you can see in the renderings and concept drawings, they would have had a single dome that would have been retractable for both stadiums, transported back and forth on rollers depending on who was playing and what the weather was like.â†µâ†µ
Capacity would have been 78,000 for the Bears and 54,000 for the White Sox -- that would have been enough for the football stadium to host a Super Bowl, but probably too many for baseball. Optimum capacity for a lot of the new baseball stadiums seems to be about 45,000 -- more than that and you just wind up with several thousand bad seats. If that park had been built with that many seats, many of them probably would have been removed for luxury suites over the last decade.â†µâ†µ
I'm not sure why this was never done -- it would have been ideally located with public transportation steps away and fantastic skyline views, rather than the monstrous renovation of Soldier Field we eventually did get. There are some other cool proposals at the stadiumpage.com link -- some stadiums similar to the proposals did eventually get built.