I am in agreement that SOU's Achilles' Heel is that is relies too strongly on a single airline for its passenger traffic. While Flybe offer a good combination of business and leisure routes on aircraft well suited to SOU they also like to chop and change a lot. Having read elsewhere that SOU will likely become an all Q400 base from this winter I sadly think the chances of the Sun routes returning in their typical sate next year is rather slim. I imagine there will be quite a few regular customers who will not be pleased with this move, especially given they have been running for the past 12 years!
Flybe may consider using Q400's for the likes of PMI and possibly ALC if it has worked out well this year, although given the jet alternatives both ex. SOU and up the road at BOH I doubt these would be massively popular, and I think we could rule out AGP/FAO on a Q400, even if it is possible on paper. Q400's could work quite well for the likes of BCN, BIA, NCE, GOA as well based on what they fly currently and have flown in the past.
A carrier like Easyjet could be very beneficial for SOU being able to operate a variety of Sun and City routes not currently served. My reservations about such an operation would be 1. Would they be happy with being limited to 2 (maybe 3 with some reshuffling) based aircraft as a result of the lack of appropriate stands for A319/20's? 2. Would they end up trying to compete more widely with Flybe, potentially killing them off routes such as EDI/GLA/AMS via a fare war and leaving those routes not as well served as they are today? and finally 3. How much would SOU affect their LGW base, and would significant expansion at the former potentially dilute some routes at their largest base a little?
BA Cityflyer I could not see operating out of SOU beyond maybe the occasional weekend charter, maybe in the future we might see a few Bath Travel charters in addition to the BOH ones they are operating next year.
Good to see that Thomas Cook will operate their own PMI flight next summer, this should be at least one of Flybe's Sun routes will still be reasonably well served;
Thomas Cook - Fri Evelop A320 (180 seats?)
Thomson - Tues/Sat Volotea 717 (125 x 2 seats)
Fingers crossed for some more good news in the coming weeks as more carries release their summer schedules.