Or an extension of the GLA- LBA to CWL
Cardiff-Leeds takes about 4h10 by train - certainly possible that this might attract air passengers. It's worth noting however that Bristol-Leeds takes 3h30 by train and doesn't have air service. I would expect that the greater commercial activity of Bristol would balance out against the longer journey time from Cardiff. One would need to have a very clear idea of corporate customers for a Cardiff-Leeds route before it can be launched
Also I would love to see a Norwich - LBA - Edinburgh service
Admittedly off topic, but Norwich-Leeds by train takes 3h20 and Leeds-Edinburgh takes 3h00 by train. Each city pair has a train every hour. Perhaps the train is just a little bit too good for air to be able to compete ?