I'm a newly recent local who would really relish an opportunity to fly from SOU over London if the option (ie. easyJet or Ryanair or Jet2 or BA Cityflyer) exists. So I am all for SOU development if it means the AREA as a whole benefits. There is no reason whatsoever that SOU can't emulate SEN. Look at the SEN arrival boards - Ryanair and easyJet all coming in well before 23:00 so that style of operation would work incredibly well at SOU with its night restrictions.
The only reason I can imagine that people on this thread are negative right now is probably down to the fact that Flybe are so godamnawful and there is literally no other airport in the country that has such a Flybe stronghold with bad service for customers, no alternatives to choose from... I avoid Flybe I don't avoid SOU.