In theory, wouldn't it be possible (in theory) to have to north/south runways to complement the current ones?
Seriously? We're having enough bother selling the idea of a new one that would be used intensely and benefit the economy?
There's already a few pain points of interaction over my house with 27 departures off London City climbing towards descending traffic on right base for Westerlies at Heathrow, the added complexities of a North South runway alignment would complicate Heathrow airspace to the nth degree, added to the fact it would cut straight across the Northolt extended centreline. There are only a handful of days when the prevailing wind isn't east / west.