"Airport" is not synonymous with "hub". Nor is "international" with "intercontinental".
I have taken an intercontinental ferry journey (approx 2 hours) and an intercontinental city bus.
Every definition has its limitations.
EWR has much less intercontinental connectivity than JFK.
Try telling that to anyone who lives near BFS, BHX, GLA, or EDI to name but a few.
Before we begin to debate the pros and cons of a dual hub model, I think we need to establish that it most certainly does exist, even if it is not the norm.
How much of an environmental impact would a new hub have in the Thames or else where? Huge! Much more than just one more runway at LHR.
A third runway at LHR might well land you back with the same problem a decade or so later.
A new 4+ runway airport should be able to handle traffic without stacking or waits for take off, so by your logic that would mean less impact. Building the damn thing and paying for it would be a different story.
nfortunately stansted and Luton are too far out, Size limited and transport limited to be major international / intercontinental hubs.
We have to look at each option on its merits for future growth, not based on what is there now. The only current London airport we can say for certain is NOT getting another runway is LCY.
You didn't mention LGW, which is surely the second most likely option if bookies were going to give us odds on each proposal (I think we know what the first is).
Any new runway needs to be supported by new terminal(s) and enhance surface access, including rail and road. The first question has to be where a new runway might go, then the terminal, then the surface transport.
What we mustn't do is follow the example of the new BCN terminal or DEN, where the facilities are built, but no direct rail link is provided.
Sort the planning out in sequence, but make sure everything that is needed is ready from day 1!