The runway will never be extended - there is no business case for it. The cost of levelling the ground would be enormous and the public and the council would never accept it because of the perception of increased noise.
Frankly, its size keeping the big stuff out is one of LTN's saving graces. Operationally, heavies would screw up runway separation and would cause a lot of taxying issues (the 76 and 78 already do when they go into the sheds). Add the vast surges in passengers crowding the less than sufficient terminal and transport links, and it'd be thoroughly miserable for all.
With LHR, LGW and BHX all so close, and STN already having unused capability, it's just not going to happen. Why so many of you seem so desperate for it is beyond comprehension. LTN is a short haul hub and does well at it. It has carved out a strong niche and should stick with it.