The only way to reduce holding is to timetable a somewhat less than 100% of theoretical capacity. I don't work in ATC but attended a talk by David McMillan a few weeks ago where he said that airlines might even be willing to do this. We shall see!
I've often wondered if it might be possible to use advanced computerisation to plot flight plans to the minute and avoid the need for holding en route but it would require very powerful computers, unified ATC and most implausably co-operation of the weather. Think what a thunder cell would do!
A third runway would help if it was not fully utilised but I would hope that all aircraft using it would park at a terminal to the north of the existing northerly runway - a Heathrow North if you like. Otherwise vast amounts of kerosene will be burnt taxiing. I am surprised that average taxi out time is at LHR is only 20 minutes but it still quicker than some airports - such as JFK (and AMS taxiing in).