The plan for a lot of that space is to become taxiway/apron space. Part of the reason that T3 is being knocked down and not replaced (and the new gates are coming on T4) is so that Delta can have a lot more taxiway room there, so that it's not one-way alleyways (as it is now), allowing planes to both come in and leave at the same time without causing even more holdups. The way the terminals are configured, building out a lot would mean knocking out some gates at T2/4. It might be possible to build out check-in areas where the current 2-3 connector and rotunda is, but that would be about it without needing a large (costly) redesign of the current roadways.