In terms of possible routes.
GIB could be a contender. Fill in the Monarch gap at MAN/BHX as well as new routes to possibly GLA/STN.
NAP is already served from 6 bases.
Jerez. Thomson used to offer MAN flights, possibly still offer LGW flights, not 100% sure but not sure what the big market appeal could be?
Seville? Most bases seem to be filling in a holiday market role. Seville possibly could t provide the mass volume?
Bari/Sardinia. The southern part of mainland Italy has remained quite sparse on the British tourist scene, with only a handful of routes to the regions from the likes of Bari and Brindisi, they too only recently added.
Podgorica or Tivat could be good beds, Montenegro really on the up and up.
One not mentioned.. Bourgas? Bulgaria seems to be on the up again, with more flights added to Varna and Bourgas from the regions, surely only a matter of time before Jet2 enter the market.