The relative speed and apparent success with which BA Cityflyer launched sun routes with their E190s would suggests there is a decent unserved market at SOU, or at least one where FR, U2 and W9 cannot compete directly due to the infrastructure.
BHX seems like a good guess too, that airport has always been under represented given the size of the city and catchment. A double-edge sword no doubt, with leakage to London relatively high.