Straight & Level -
EZY already fly Luton to Palma, Barcelona and Madrid, Monarch fly Luton-Gibraltar. And I reckon there will be LTN-Poland flights soon.
Cyrano -
Where should Duo fly from Luton?
Well, how about Stuttgart? Already a Duo destination, and Stuttgart-based Mercedes-Benz's UK head office is in nearby Milton Keynes. To say nothing of other commercial links.
Lisbon is a European capital that has attracted some Lo-Cos without great success (Go and AB). So how about LIS?
Vienna, Oslo and Stockholm are other possibilities - no LoCos and all are European capital cities.
Rome isn't served from Luton although EZY and FR both fly there from STN. There's a service from Luton to Milan (Bergamo), but not to Turin.
Cork hasn't been served from Luton for 10 years, and Ireland is enjoying something of a commercial renaissance.
Strasbourg, having lost its FR flights, could be a possibility for the Eurocrats, as could Luxembourg.
At this rate, Duo will become bigger than EZY!!