Jabird, suspect that the long term plan is probably to link the transit from Heathrow 5A/B/C to a potential "5D" and to a rebuilt Heathrow 3 (particularly for BA/oneworld transfers) and eventually to the new Heathrow1/2 and its various satelites.
A parallel landside route including the central bus/railway station would also make sense. It would certainly be far more convenient than ferrying pax around on a fleet of buses. On the other hand this may be just a bit too much joined-up thinking, or just too expensive.
Barling Magna, your comments about domestic turboprops such as Flybe using NHT to restore thin domestic routes that used to operate out of LHR could be a "permanent" temporary arrangement, until such time as LHR gets its two extra runways (could be a very long wait regretably).
Could NHT be a shared military/civil airport, like Newquay and Frankfurt used to be, and Don Muang (Bangkok) still is?
NHT is limited in growth potential so any monorail over the A312 between LHR and NHT would need to also link in to existing rail and underground lines at both ends and to the HS2 at the Northolt end.
Anyone notice the pigs flying overhead!