The best London airport is Stansted, hopefully all expansion will take palce there. Despite doubling the terminal space, it is already running at capacity again in the check-in area in rush hour, and it could really use another runway, some more plane stands and even more check-in space...
Luton, on the other hand, is a badly designed airport (as pointed out above). The check-in area is of a poor layout, the main terminal works on the same principles as Heathrow (i.e. cramp all people into a small central space and tell them which gate to go to at the last minute) which is annoying and uncomfortable, the planes have to taxi onto the runway, drive to the end, and turn around, something acceptable at LCY
and other low-traffic airports, but surely a cause of delays at any busy international airport.
What Luton could use: A new terminal, just replace the entire dreadful current one. Some taxiways to the very end of the runway. Improved connections to central London (i.e. a train station inside the airport, and express trains that don't bother stopping at every little hamlet in between).
What it will get: Nothing, probably.
With luck, Heathrow might get the extra runway and Stansted might be allowed to grow. I strongly doubt they'll grow more than 2 airports simultaneously, and Luton just isn't worth bothering with... (says the armchair manager in me)