User List Field Based on Another Field Value?

I can think of one way to do this, but it's a little bit klugy. You can make multiple list user fields each with a custom user set. These would correspond to each location. Then you could have a form rule show which one to display in edit mode.

Then what you can do is have a formula list user field that would combine all these into one field for reports and view mode on the record.  

But you would need to maintain those multiple list user fields