I don't for a second believe RYR will keep their promises of major expansion at LDY after the completion of the runway extension. Faro and Alicante.......fair enough. But why the constant messing around with the rest of the schedules? Daily LPL and PIK during the winter, cut to 5x week and 4x week respectively during the summer? Where's the logic in that? STN had proven itself as a successful route over the years, but even it has not been safe from the meddling.
Dusseldorf Weeze (i.e. nowhere neat Dusseldorf!), would probably work from Derry as it would be a European route with inbound pax from Germany boosting usage. The other routes you've listed Amelia would obviously work too.
It STILL isn't rocket science. However, in my mind the priority should be meeting the demand for LON routes by increasing capacity on that route.