My computer could in principle tell if I'm at home or visiting parents depending on the MAC of the wifi I'm connected to. It'd be nice if I could tell the weather plugin "change my location to Parentville, but only when I'm on this network. Otherwise, I'd like it to be Hometown as ususal".

Maybe the settings could look like  
  

  [Locations] (Units) (Appearance) (Scrollbox)

  Current:
  *Hometown, Mycountry*

  Stored locations:
  | Hometown, Mycountry (*default*)         | [Edit] |
  | Parentville, Mycountry (Parentrouter)   | [Edit] |

  [Add a new location for this network]


And the Edit button brings up the old dialog, e.g.:
  
  Location: [ Hometown, Mycountry      ] [Change]
  Latitude: […]
  Longitude: […]
  Altitude: […]
  Timezone: […]
  Currently the default location.
  [Save changes]
  
or, for router-bound locations, Add/Edit gives: 
  
  Location: [ Parentville, Mycountry   ] [Change]
  Latitude: […]
  Longitude: […]
  Altitude: […]
  Timezone: […]
  Location bound to router "Parentrouter" (00:22:A6:B2:C0:00).
  [Make this the default location]
  [Save changes] [Delete location]