Unless Jet2 is prepared to do a 'Gill Air' and ferry an aircraft back and forth from the City to the Intl i cant see Jet2 happening (though happy days if they did jump ship). Or keep 1 aircraft in each airport? Either way it sounds a bit too messy and expensive to happen.
If the City Airport doesnt already have any airlines lined up to start soon, theyre certainly still trying to get them in. Quite a few big cheeses in recently, and the airport has been tidied/freshened up. Though the story from management is that the airport might also be going up for sale or looking for new investers.
As for airlines that may be starting, I can vouch for 2 that have declared interest in Sydenham, but we'll wait and see.