Of course. But that said I'm not sure about the C919's prospects to be used for AEW, and the CR929 isn't coming anytime soon... I doubt we'd be allowed to modify Boeing or Airbus aircraft for AEW or tanker missions within this decade either. Given the limited amount of Il-76 tanker and AEW variants we have in inventory, wouldn't the Y-20 serve as a good stop gap until we can find a more ideal airframe?
True, it does not look likely there will be a better platform than Y20 in the next 10 years to come. It's not a great platform, but it's the only one out there that can do the job to satisfying degree. (the accent on the smaller Y9 as main AEW platform work shows just why is Y20 too big for some roles)
CR929 is likely not going to be ready before 2030 and even then it's not guaranteed all parts are going to be domestic, thus allowing for the airframe to be used for military purposes. Plus, CR929 is really too big, definitely for surveillance/elint/ew, and even for aerial tanker purposes it's a bit over that sweet spot, so it's questionable just how much more efficient it'd be over a Y20 tanker.
CR919 is on the cusp of being ready, but it suffers from non domestic parts which is likely going to preclude it from military service for the next 5 to 10 years. And even then, tanker variant would be too small, a worse option than Y20 tanker. For surveillance/elint/ew though it could be just the right size. Some day in the future when it's fully domestic.
Which is why i am sometimes questioning the validity of making a dedicated Y20 based plane that'd not be a transport but would serve other purposes. Of course, it'd most likely be too expensive to develop for the number of airframes needed... But if for some reason China does want 200 tanker/aew/other platforms in the next 10 years (unlikely, i know) then it might be worthwhile.
It doesn't need to be fully optimized, as then one would really talk about a commercial airliner. So the wing, engines, the tail section and the nose would likely stay. Even the landing gear could stay in good part, minus an axle or so, as rough runway requirement might come in handy for strategic assets. But not optimizing the fuselage structure to hold 50+ tons in one small area, not making it so wide, that's bound to shave some tens of tons. On top less drag. And high positioned wing could even allow for some useful sensor placement on the fuselage, in certain variants. landing gear nacelles would still protrude quite a bit, but as said, optimization would have its limits when it comes to cost effectiveness.
... but I would say this is it!