Jet2 have one new base to open next year, but it seems to be a tightly held secret as to where. They have a significant number of slot allocations in Spain, Portugal and France which don't tie up with existing operations and, if the fleet plan is to be believed, around three 737s spare.
That said, I would not have thought Cardiff would be a particularly good fit with their "The North's low-cost airline" mantra. Most speculation seemed to be pointing to Durham/Tees Valley as the likely place, since they were apparently a bit miffed by FlyGlobespan's move in there.
Rumour has it that at least one long-haul operator is looking at pulling out of CWL completely given the outrageous landing charges for 767s. CWL sounds to be amongst the most expensive airports in the country for that size of aircraft given the way its landing fees are set up. It will be interesting to see if the airport budges on this for the sake of keeping services going.