Never say never but I'm not sure. There's a market from CWL which they could take advantage of but there are also many unserved airports in the UK which may prove to be more profitable. Having said that there is increased competition at these airports.
There is a rumour that a loco is coming to Cardiff next year. It could be Jet2 but if I had to put money on anyone it would be a few more rotations from FR or VY. Not necessarily a base but increased portfolio of destinations or frequencies.
While it would be great for the airport to have a based airline, I hope they don't put all their eggs in one basket to the detriment of the offering of other airlines. FR flying to ALC AGP etc would be the death of VY at Cardiff. They should concentrate on increasing passengers be it from a based airline or visiting airline in order to create choice and competition.