As I have said many moons ago, Luton has been a building site even since the 80's and will continue to be one until they knock it down and build something new. Add ons here and there sound good at the time but don't last long, so they add on more and more until such time the whole airport becomes a mess.
As much as people in this thread knock Stansted, at least they had the sense to leave the old airport alone whilst they built the new one. Granted, they had more space at Stansted and maybe there lay the problem for Luton.
It just doesn't have the space to be developed into anything major.