They could do london city as that is where their main flights go from. Or Ireland and France i.. Nice would be Nice lol.
As Cityjet is a business airline Nice is pretty much out of the question. They only operate to Nice from the business centres of London City and Geneva. Possible new routes from Cardiff would be Dublin, Paris or at a long shot London city. However Cityjet are pretty short of aircraft at the moment so I'd say any new route would be a year or two away.