Fascinating! Can't guess who it could be. I can think of good reasons why it wouldn't be any of the people I can think of!
BA... not in line with their way of operating; slots now available at places that are.
Jet2... already up the road.
BMI, Eastern, Loganair... not their kind of airport, not their size of base, not their number of destinations, no potential for their types of destinations (this one less applicable to BMI, but still enough so).
CityJet; would be a hilariously ironic option, but not British and giving up on that sort of thing.
FlyBe... not likely; not expanding.
Other holiday companies... too big a base and they're unlikely to want to fly suitable aircraft for it.
So Jet2 after all? Or someone new? Or the tremendous coup of someone that's at LCY?