Why not civilianise Northolt and re-align the runway to suit air traffic routes in and out that dont clash with LHR. Link Northolt to LHR by a tube extension and Northolt becomes the runway for regional traffic.
Less land to grab, less planning problems, increased use of an under utilised airport infrastructure.
Not ideal, perhaps. But then no solution to the need for a brand new hub airport somewhere around about Oxford to suit both London and the Midlands is!