If the third runway is built, it will be in the commercial interest of the current mainstream operators (especially BA, but also BMI, Virgin, etc) to fill it up as rapidly as possible. Smaller aircraft and more frequencies on established routes is the best way to do this. This is the principal way to prevent competition from coming in to your existing territory and getting a proportion of your market.
Likewise if you are a operator who does not serve Heathrow yet (for example the few remaining mainstream carriers who work through Gatwick) then be in there from day one of the new runway. Leave it a moment longer and you will find all the slots have gone again.
This only works at a few places; you could not for example do it at Teesside, you would never get the revenue to cover the costs. But it is the only way to work at Heathrow.
The only way holding will ever be reduced is if the airlines don't all arrive at once.
Not really practical because the airport is up to its maximum runway capacity for both arrivals and departures hour after hour all through the day, and has been for years. It is not a "peak and trough" situation which can be rebalanced by smoothing things out, or like a US hub operation where they all come in together, then depart together.