Personally i cannot see Jet2 setting up base at BRS, too much competition on the routes that they would seek to offer and ramp capacity issues for morning aircraft especially, pretty much rules out a base.
CWL for me would be the natural second option after STN (which i presume is the front-runner in the rumour stakes at this moment in time). Still a relatively untapped catchment since the demise of Baby all those years ago, many routes still underserved on what they once were, plus an article today has highlighted that the Cardiff City Region is 'Booming' at present.
Other options could be BOH, SOU, although the former pretty much serves its catchment well enough, with a pretty decent FR presence at the peak of Summer. SOU - relatively limited runway may hinder some of the routes, plus this serves its catchment quite well too.
For this number of jobs to be created i'm assuming there will be more than 1 new base opening in the next year or so.