Maybe when TUI completes their HQ move from London to Luton more development may come from one of their airlines. I'm sure HLX could give a german route another crack of the whip again, and am quite positive with the right advertising it would be successfull, as I was very surprised when they pulled Cologne a couple of years back.
Also, ThomsonFly, the low cost branch, have been quoted as saying they can't afford not to be in London, so surely LTN would be their natural choice, unless they choose to develop an airport, similar to what they did with Coventry, meaning Biggin Hill perhaps?