The JASDF has some Boeing 767s with the E-3C radar & associated systems, so in theory it'd be possible to migrate everything to an airframe that's in production (for a while) & with which the system's already integrated, but I don't expect anyone will actually do that.Honestly I would not be surprised if at some point in the near future the USAF were to start looking at augmenting/replacing their E-3 Sentries. The base airframe for the E-3 has been out of production since 1991 and it is now about 27 years later. At some point, the cost to support and sustain the aircraft in a flightworthy condition, plus keeping the avionics and mission systems upgraded to an operationally useful state, is going to be more than replacing the E-3 Sentry with new units with modern capabilities. Given that the NATO E-3 Sentry force has started to retire some of their AEW aircraft due to the cost to maintain them in light of their accrued flight hours, it would seem likely IMO that even with some of the block upgrades being done to USAF E-3's that threshold will be met.
From there, it would seem logical that the US would look to domestic airliner designs which can fit the avionics and mission systems required as well as provide the power generation, range, loiter and mission endurance times required. IMO it would probably not be one of the current B737 NG designs like are seen in the Wedgetail or Poseidon, but one of the follow-on B737 designs.
I agree with those who think that for the UK the most likely choice to replace the E-3 is the B737 AEW&C, in the absence of a US national project to join. A SAAB system, whether on a Global 6000 or an Airbus platform, is definitely an outsider.