That would be the likes of INV, JER, IOM, GCI and the possibility of adding MME, NQY and maintaining LBA. This would allow LHR to directly benefit the regions with one stop to the world and would be served by modern and quiet aircraft.
As for Gatters, look at the big players. Under no I would say :
flybe (unless they introduced the routes as per above)
Fair enough, "will" and "overnight" might be a bit strong, but remember all the assumptions regarding new capacity are that the existing airlines will want to add new routes, and that other airlines will also want to come in to the London market.
Virtually all the airports you mention above are indeed already served from LGW, therefore the opportunity to serve LHR and to benefit from that connectivity would no doubt be taken up if it could.
Or, we allow LGW to be the one doing the expansion, in which case BE evolve further as a network carrier. Remember, they are so far the only airline to have expressed any interest in an island airport, and they have done so for the very reasons you mention.