Perhaps in the world where you (the red-teamer) sets up their phone and/or laptop as an unencrypted/open wifi hotspot access point and then follow them (the blue-teamer) to their favorite coffee spot / burger bar / etc?

If I recall correctly even current phones will connect to open wi-fi spots preferentially and/or automatically. Bingo, job MITM done! Bonus points for having a tool on the red-teamers' laptop that can send wi-fi de-auth packets :)

That would be the first thing I would look in to to see if it is still do-able today if the problem was 'hmmmmm. Given the parameters, how could I MITM the blue-teamer?'

I'm sure that others can come up with even wilder ideas involving can-tennas or bird-dogging the blue-teamer into a elevator with a 'running useful and interesting stuff' laptop in a backpack and wait for the blue-teamers' cell phone to start reaching out desperately for a way to remain connected (cell tower, wifi, 2G cell signal etc) either of which might work