There is a CTA Blue Line station just south of CUS on Clinton Street. In addition, there is direct service beween CUS and O'Hare on the METRA North Central Line but it only operates during rush hour.
Current plans are to make use of UP tracks to/from EauClaire, but not north of Milwaukee. Service to Green Bay would likely run on CN (ex-SOO) trackage north of Duplainville.
Current plans are for Hiawatha Corridor to expand to 10 trips daily. Four trips would continue to Madison, three to Green Bay, and three to Minneapolis. This is from AMTRAK 2021 Corridor Vision report.
Service to Janesville and Madison is among new routes being considered, but it would require restoring several miles of track south of Madison. METRA is looking at new regional routes to partly make up for lower ridership post-pandemic. Service on current UP lines could easily be extended to...
METRA needs to keep Ogilvy and LaSalle Street open due to volume of trains. Current plans are to re-route SW Corridor to LaSalle which would create more openings on South side of CUS.
The O'Hare people mover has been extended to the rent-a-car garage located next to METRA O'Hare Transfer so there could be express trains between CUS and O'Hare. There already is a CTA Orange Line from the Loop to Midway airport, but this route by-passes Union Station.
There have also been discussions about eventually running AMTRAK Michigan service over South Shore Line between Kensington and Michigan City in order to avoid congestion on the current NS route.
AMTRAK is probably happy to let Metra operate "regional" service like this up to 150 miles. There are plenty of AMTRAK corridors where added frequency will require extra equipment.
Metra is also considering other "regional" service to nearby cities like Madison, Peoria, Champaign since ridership is still down from pre-pandemic levels.
The closest stops on the California Zephyr would probably be Mt. Pleasant or Ottumwa. Service between CUS and Moline should start soon with 2 trips a day.