BM
I think it would create a great opportunity for British Midland Regional to set up a base at Southampton especially as they are working closely with Lufthansa and code sharing on several German routes mainly from Munich.
British Midland are on expansion mode at present and have the suitable aircraft in the EMB 135 for ex SOU. Although not a member of Star Alliance British Midland Regional could offer great connections from Southampton to the following hubs such as
Brussels
Copenhagen
Frankfurt
Munich
Zurich
I can't see Flybe serving such markets having tried and failed previously but surely there is local demand for such routes.
The new owners of SOU should be able to secure at least a few of these destinations and possible new airlines as long as they sort out the current apron stand situation.
Other options could be the return of
Vueling to BCN and possibly other Spanish destinations
Aer Lingus Regional to ORK
Air France to Paris CDG
KLM to AMS
And maybe
Air Nostrum 'Iberia Regional' to MAD