There are multiple directions that this would go, but here are thoughts from the discussion at https://talk.openmrs.org/t/limiting-user-accounts-to-login-locations/8672/
the login screen just has username/password. If you successfully enter those, the next screen can show you available login locations (and it can use user details to do this, in addition to the current cookie value for last login location). Once you choose this, you're then taken to the home screen. (To keep the workflow fast, you should be able to select the most recent location, or the recommended location, just by pressing enter.)
Mike Seaton preference was actually to just take you to the home page without specifying a location, and from there you can optionally set one, but some apps could be accessed with no location.