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I don't know what the landscape looks like in Chicago for WalMarts, but poking at Google Maps shows there's potentially 9 to 10 Supercenters on the chopping block. Each one is around 10-11 acres in surface area, including the parking moat.
OP's got the right idea. All of these should be replaced with a no setback, no surface parking, underground only parking like Europe's been using for modern construction, mixed use high density housing and stores neighborhood. Compared to money pit the WalMarts are for the city, turning them into real density would help Chicago keep the suburbs growing. None of the WalMarts are downtown, of course, but they're usually already next to roads with at least bus transit stops, and by making them into serious 4-5 story housing blocks with no parking moats around them they'd quickly become candidates for streetcar or metro line nodes.
This is a real opportunity for Chicago to improve city land use. Every big box store that closes should be converted to a dense mixed-use walkable neighborhood if cities want to start reclaiming their land for people instead of cars.