Flybe already operate a SOU-LPL route three times a day now.
The only way BOH would be able to get enough domestic volume to make any impact - because of SOU's adequate hold on high freq domestic routes - would be with easyJet (GLA, NCL) Ryanair (LPL, BHD, INV, EDI) and Jet2 (summer weekend services from BLK, MAN, LBA much like NQY manages to attract)
EASTERN airways already operate from SOU in a big enough way to prevent them from looking seriously at BOH. Even if they got pi*^ed off with SOU/Flybe; moving to BOH would not solve their problems - there is no business traffic; they would effectively be shrinking their catchment.
AIR SOUTHWEST would potentially be good on JER, GCI, MAN but depends on the deal the airport can give; i doubt they would base a/c which means only w options. I would like to be proved wrong here though.
I do believe there is potential for a new leisure focused 30-50 seat start-up so long as it has code-shares for onward connections, affiliation with or franchise with a major brand and operates at good timings and adds routes like JER, MAN, BHD, GLA, FRA, EGC, AMS, CDG. But one major problem - how to persuade inbound travellers to use the flights without the noise created by Low Cost Carriers? Ryanair have no problem pushing inbound into BOH.