So GIB looks like a good contender.
Based on the distance from LON-GIB at approx 1,750km, Similar distance/ unserved destinations potentially on the radar include
Seville / Jerez - Spain
Naples / Bari / Sardinia - Italy
Podgorica - Montenegro
Depending which way you twist it, could be a potential case for any of these.
Or we could be jumping the gun completely, with it being extra capacity on similar routes...