If STN is to become the next Jet2 base, after that further options are limited if the airline wants to cover the entire country.
Others have already pointed out the pros for CWL: it is underserved in many of the popular sun routes (especially), and the fact that the Wales government has provided its airport with a multi-million pound routes 'war chest' loan might also be a positive factor.
If Jet2 did go to CWL a two-aircraft base would seem to be a likely beginning. bmi baby showed there was and no doubt still is a market for sun routes, but this would obviously lead Jet2 into competition with Vueling which itself has gradually increased its summer sun route frequencies, as well as with TUI and Thomas Cook.
Jet2 would still be missing out on the generally well-off West Country market but might judge that existing airlines at BRS have the market there covered and, anyway, they might be able to attract some passengers from there to their new BHX and CWL (if it came) bases.
It might be worthwhile looking at what the Jet2 CEO is actually quoted as saying which is: "We have exciting plans to expand the number of destinations we fly to, as well as the number of UK bases we fly from, and this means that our team is growing all the time too."
Given that airlines and airports are arguably better than Shane Warne when it comes to spin, the allusion to expanding the number of UK bases might simply mean BHX, which has been announced but not yet started, and an as yet unannounced base which many believe to be Stansted.
If they intend going beyond that I would certainly not bet against CWL.