I find it very difficult to believe that no airline apart from Ryanair, Easyjet and the odd beach route airline would contemplate using Stansted. It is after all a large well-equipped airport with a long runway, large terminal, capable of handling a wide variety of operations, and good ground transport links.
I suspect that under BAA ownership, there is political pressure within the company to encourage major airlines to look only at Heathrow. At the very least, with the competition appeal process dragging on, Lufthansa opening a Stansted-Frankfurt route as an example would be seen as highly undesirable to Ferrovial's overall aims.
Remove this ownership / internal political pressure restriction and I suspect the business development / marketing team at Stansted could be rather more creative in their propositions to new airlines.