The main issues at BHX are the poor planning of their late aircraft deliveries which basically shot their schedule to pieces. Hopefully lessons would be learned and once deliveries arrive things will improve. Also the way they have handled the situation afterwards hasn't painted them in a good light at all. That being said, one would hope they reign it in and look to improve things as things settle down. From what I've read the CDG and STN operation has been fairly stable, with the exception of the National 757 reliability issues.
Would they be good for CWL? Possibly, if the operation ups it's game and improvements are made to Customer Service.
They do mention that BHX is still going to be happening in 2019, along with STN ops and a growing CDG schedule (LH)
Other forums are slating that Primera won't return to BHX as they've burned themselves out completely and may not recover from that. That could well have impacted the Wales and Southwest catchment they may have used them at BHX. Given that Norwegian have certain areas of the UK and Ireland covered from a low-cost long haul, with the Midlands, South Wales and Southwest pretty much uncovered, it would be bizarre for them to switch further South.
The question would be, BRS or CWL? BRS for catchment, CWL for operational capability. I would be interested to know how the A321 would perform off BRS' runway.
As for connections, I can't see any legacy Airline that would come to CWL given the reduction in UK regions so options would be limited. JetBlue would be good, but they are rumoured to be interested in premium heavy routes.