Obviously LBA & MME have been & gone from LHR, as the routes became unviable for various reasons, including the challenges of making optimal use of LHR's slots.
Problem is - build a new airport, and you still have to deal with the huge increases in PSC that would be needed. So the thinner routes would go from being unviable due to slot restrictions to being unviable due to the cost (in addition to all the othe challenges)>
Where's the gain?