ORD to MKE is a 60-90 minute drive depending on traffic. When you account for going through ORD security, boarding, taxiing at ORD, etc. I'm not so sure that it's a win for travel time. Depends on where you're starting from in the Chicago area, I suppose.

Your DEN to COS suggestion would probably be quite popular for skiers coming in that want to avoid renting a car and would enjoy the electric hop to avoid a 2hr drive.

The real winning route would be two airports separated by ~100miles that still have a 3+hr drive, or two small regional airports where security time is negligible (that's certainly NOT O'hare, in my experience at least lol) Flights leaving GRB to northern Michigan would be perfect for this though since the alternative is to drive around Lake Michigan.

ORD to MKE is a (mostly) silly flight if that's your whole trip.

ORD to MKE is reasonable as part of a X -> ORD -> MKE one stop; at least when I was using MKE, I'd always need a stop (Midwest was too expensive for me) and ORD was an option, although I think I always went through DFW or PHX.