Mass airline passenger imbalance.
Major world air carriers have created a major imbalance for international travellers by converging into LHR and not into an international hub in Continental Europe.
A major international hub for Europe needs to be created at CDG or FRA; reducing the traffic into LHR which is located on an small offshore island from the Continental Europe.
This would reduce congestion at an old two runway airport such as LHR is by not having to shuttle passengers over to the Continent.