I don't think Jet2 will jump seriously into the city break market from any base as things stand. They are primarily a sun/ski route leisure airline not far removed from TUI and TCX IMO.
This leaves BHX in a bit of a hole as far as unserved city routes like ARN, LIS etc as I really can't see any suitable carrier to serve these markets. We all know that an EZY base will never happen (even if it did I doubt ARN would be their first choice of route!)
Jet2 may well diversify in the future but for now at BHX I feel future short term growth lies in leisure short haul (Jet2 TUI TCX) with legacy carriers static (apart from an outside chance of Finnair?) and long haul scheduled staying reduced to Mid East and India/Pakistan.
Its a very different outlook to just 3 years ago when you could barely keep up with new routes and we had a brief sniff of China!