The city already published a document that includes details what the long term possibilities/plans are for ORD.
https://cst.brightspotcdn.com/cc/34/ce66...ior-lien-revenue-bonds-series-2025eg.pdf
Before we get a Western Terminal, there are a number of other things that happen, probably in this order:
Satellite 1 is under construction. Then new the Global Terminal. Then Satellite 2 is built. Satellite 3 can proceed without a new airline approval vote once O’Hare has handled at least 101.5 million annual passengers for three consecutive years.
Then add Satellite 4, which would acquire airline approval.
After that it's more speculative, but a northward expansion of the currently under construction Satellite 1 is likely, adding gates there.
A complete redevelopment of Terminal 1 and the B and C gates there.
Redevelopment of Terminal 3 and concourse G.
There would be an additional parking garage and ground side Terminal 5 improvements happening in there somewhere.
Once there are three satellites and a people mover, it changes what's possible on the west side. A terminal there would *probably* not have gates. It would be access and a passenger processor plus parking and ground transportation on the west side with a connection to the new train. Of course, by whatever decade this is feasible we'll probably have AI piloting fusion powered electric aircraft, so who knows what security and flying will be like.
Anyway, with the ability to add three more satellite terminals plus the expansion of D, we're probably talking about ~70 more gates being possible. With a reconstruction of T1 and T3, we could accommodate a lot more widebodies if the satellites were handling smaller jets. Only Denver, Dallas and Washington Dulles have the full combination of land, runway geometry and development space to contemplate expansion comparable with O’Hare. Denver has preserved space for 12 (!) runways and sits on 53 square miles. It is probably the only airport whose theoretical ceiling for passenger capacity is higher than O'Hare's.