Starlux said they are focused on U.S. market for now but will eventually go to Europe. Although they are very much marketing themselves as a high quality top level airline and seem to have a lot of money so I suspect they would pay for LHR if needed.
I can’t see United at LGW. They have a maintenance base and huge lounge at LHR plus their partners. It would make no sense to add a random once a day to LGW.
AA won’t be back either. BA is still short of frames and I would imagine AA could take over BA routes if needed like the do or use BA’s LHR slots of which they have plenty of slot sitters.
I suspect Etihad will cough up and pay for more LHR slots if they want to increase frequencies just as they have done in the past.
I would guess that further long haul growth will come from India, China possibly Thai, and African carriers.