At this stage it is almost too late - it is coming to a point among travellers that Heathrow is no longer a transiting hub - too stressful an experience. It can't be expanded any more, despite that not making business sense. Another runway would ease congestion but as has been mentioned, if it was EVER built, BAA would fill it up to the brim once again, intensifying the problem.
Eventually demand for regional flights ex LHR will drop as consumers lose interest in flying to or through LHR. It is much much easier to fly via DXB or AUH, or CDG or FRA than LHR. At best I see it as becoming the final destination, competing only with other London airports for airlines. Etihad for instance has opened routes to many cities which normally you would have to connect in Europe to access first - e.g. Kiev or Almaty, thereby removing the necessity to transit in Europe.
I realise I am talking about a very specific part of LHR's userbase - i.e. those on the Kangaroo route. I regularly travel from Australia to Ireland, for instance, and what I've noticed is that AUH-DUB is almost always full. There definitely seems to be demand among consumers to avoid the congested mess that is Heathrow and fly direct to the destination, even if it means paying a little more for a ticket.
Middle East hubs are becoming very viable and in my opinion Heathrow's days as a transit hub are limited. Therefore, I think the wisest option would be to develop other London area airports and upgrade the rail links between them. This maximises the number of ways pax can use London airports in transit and eventually means more money overall. Concentrating on milking one airport is doomed to failure, which can be seen on any day at Heathrow.
But those are just my 2 cents and I'm sure I've overlooked hugely important detail.