what One World might pay one of the other alliances to quit Heathrow, or what might be demanded
I think that would be deemed a bribe by the various competition powers.
The only hope would be that a dual runway LGW could offer one of the networks (assuming VS are fully in Skyteam by then) a huge midfield hub to their own specifications - but even if they gave it away (say for 10 years), LHR is where they want to be, and reducing hub costs wouldn't make up for lost yield.