I would have thought that LTN-DUS is an obvious choice as there is no local competition (Although then would they drop the Dortmund service and concentrate on DUS?).
I would be very excited about a FRA service as well, I hate having to fly into Hahn on Ryanair, would much prefer EZY and from Luton would be a huge plus.
Could all be a pipedream though, we can dream...