The Y-20 would more likely to primarily satisfy the strategic transport needs, then branch out into other purpose - though I'd believe that the Il-76 airframe rotated out of the transport role would be converted to those platforms, thus streamlining the logistics.
But ultimately, AEW / maritime partrol / aerial tanker are better with using airliner airframe, given their typical operation is to fly at altitude in "race track loop" pattern, which would be a waste of airframe potential in the likes of Il-76 or Y-20, alas previously PLAAF has no better choice to do so - however, by the time Y-20 fleet become the fleet's mainstay, there should be enough Il-76 airframe for further conversion, and C-929 project should be progressed further enough to be given a try in military roles listed above.