Flybe
I agree with you and BCN is long overdue from SOU.
I would like to see the return of Cork and Dubrovnik as well as Hamburg being added and maybe a Scandinavian destination but I could see BCN, IBZ, MAH, MPL, MXP, INN, WAW working.
Malta would be a good destination but it would probably work better with a charter flight rather then a scheduled flight.
Previously Air Malta operated charters from SOU on behalf of Thomson Holidays and Newmarket Holidays, they were popular although I think some were routed via Exeter!