Arrival often includes several minutes observing the Essx countryside whilst stacked in the hold.
Surely the whole idea of building a 3rd runway is to increase the arrival rate? This alone would massively reduce wasted fuel and CO2 emissions. From the plans I have seen it would be more of a small satellite airport to the north rather than a 3rd runway for the existing airport.
The key issue would be to restrict maximum slot usage to between 75-80% for example of the theoretical maximum so that the current saturation problem is avoided in future.