For new routes, I would bet that Bulgaria must be on the cards. Bourgas seems to be a big missing link in their network, especially with Jet2 holidays.
As for other routes, could Sicily be on the cards? Seems to be a decent trade in holidays to Catania so perhaps one Jet2 could jump in to? The only o5er routes I could see are perhaps another Greek island like Mykonos or Santorini? Hotel Capacity might be a bit tight on those islands though...