Due to access difficulties, which will be even worse when the runway extension goes in, suggest that any significant development in the northeast corner is probably pie in the sky unless part of the rail yard can be procured. Unless this can be done a tunnel would be prohibitively expensive, partly due to a major gas pipeline running east/west across the runway development area (no mention of any tunnel in the runway extension planning application) leaving surface access the only option.
Due to proximity of the runway surface access would need to be controlled, only possible between aircraft movements. This issue came up many years ago when a previous attempt at development in the northeast corner was being planned. Planners indicated that ATC could control traffic lights, to which a very short reply was forthcoming, second word being “off”!