Heathrow barely functions well on a good day. Only because of an incredible, almost super human effort on an hourly/daily/weekly basis by the operations staff does it function at all. It's status in the world will continue to diminish because it is incapable of being expanded.
But let's ask a simple question first. Why does Heathrow exist? The answer is simple, it is an airport. Everything else is ancillary. It is not a shopping centre, it is not a parking franchiser, it is not a food court but rapacious management appear only to focus on these elements. The operations side of the organisation, the bit involving aircraft, appears to be treated as an inconvenience. But if we accept that an airport is place where people arrive and depart by aircraft and not a retail opportunity, then we can make some progress.
So let's put in a measure of success. For passengers using an airport as a terminal, I'll suggest that this should be a measurement of how long it takes you to get from where you start/finish your journey (home, business, hotel, resort etc.) and the aircraft being just airborne. For transfer passengers is should be measured from landing to airborne. Hence proper high speed public transport connections, remote and travelling check-in and freedom from on-site parking.
Location: Yes, on the west coast mainline.
On-site parking: None. But there will have to be a proper 24 shuttle from staff parking to their place of work. Having multiple remote sites will reduce the impact of engineering works and mechanical hiccups.
The operator runs the remote terminals, possibly in conjunction with other retail centre operators. But make these places public access. So instead of having an airport with shopping centre, have a shopping centre (or two or three) with an airport.
Liverpool and Blackpool will be able to close as well.
It will pay for itself but I'd expect a lead from a government. Remember, LHR (together with LPL & BLK) will be sold off with planning permission say for lots of lovely "affordable housing."
Carriers: Any who want to use an efficient airport that will give them access to easy transfers and connections to the rest of Britain.
Will people go? At the moment, millions of people travelling to and from the UK via CDG, AMS, DUS, DXB and regional airports because they can't use LHR. If your measure of performance is time door-to-door, then Britain (or what's left of it after Jockland departs) and London will be better off.
That LHR is spending money doesn't surprise me - probably on more shops to further increase the retail opportunities for its poor victims enduring yet another delay. Quite what it will do for the place on a foggy day at peak hours is not so clear. But there is no vision for the future, just more if the same.
Will it happen? I doubt it. But what is certain is that longer LHR stays open, the more certain my future becomes.