I've been playing around with some numbers, trying to work out what kind of specs a Y-20U tanker could have, specifically being their max fuel capacity
For comparison, the Boeing KC-767 has a max fuel capacity of 72,877 kg. The 767-200ER it is based on has a fuel volume of 90,770 litres
Il-76 has a max fuel capacity of 88,000 kg and a fuel volume of 110,000 litres
In both cases, I'm only including the aircraft's inherent fuel tank payload, not any additional cargo hold tanks.
Il-78, if it lacks cargo hold tanks, has been cited as only being able to cite 57 tons of its 88 tons of fuel. With cargo hold tanks, the transferable fuel reaches 85.72 tons, and with the Il-78M variant where cargo hold doors and the transport function is removed, it can transfer 106 out of 138 tons.
KC-46, a derivative of the KC-767, has a max fuel capacity of 96 tons and often cited with a 94 ton transfer capability. I haven't found similar numbers for KC-767 unfortunately.
I would surmise that some of these differences in transferable fuel is due to Il-76/78 having less efficient engines than their newer commercial derivatives, but I suspect a lot of it also depends on the unspoken mission profile the aircraft will be expected to perform. For instance the KC-46 94 ton out of 96 ton fuel transfer capability almost certainly is not for any kind of long endurance mission orbiting 2000km from base.
So what does this all mean for Y-20? Well, I think it is reasonable to expect Y-20 to have at least an equal if not greater maximum fuel capacity compared to Il-76. And all Y-20s using WS-20 should also have significantly lower fuel burn compared to older Il-76s as well.
With a 220 ton MTOW and a proportional fuel capacity, Y-20 should be quite competitive with the KC-135, KC-767 and KC-46 offerings in terms of absolute fuel capacity, even without cargo hold fuel tanks, allowing it to perform the same transport functions as the other planes.
Actual transferable fuel may be a little proportionally lower due to having more engines, due to slower probe/drogue fuel transfer times compared to aircraft using boom/receptacle methods.
Y-20Us equipped with cargo hold fuel tanks will provide enhanced transferable fuel, and removing things like cargo doors and cargo handling equipment will enhance it even more -- but at the expense of cargo transport capability.
So the key point I'm getting to, is that the PLAAF should look at equipping all standard transport Y-20s with the ability to conduct aerial refuelling by adding associated refuelling pods, machinery, and electronic supervisor cameras/equipment. It will provide a relatively capable aircraft with dual transport/tanker functionality (even if its transport volume is less than say a A330 MRTT due to its military transporter cargo geometry). More importantly, it means for every Y-20 inducted the PLAAF gets a tanker as well.
This doesn't preclude the idea of introducing a Y-20U variant with the transport doors and machinery removed entirely to increase the transferable fuel to 100+ tons, but it does mean the PLAAF should look to provide a distributed, large number, transport/air refuelling capability for its fighters supported by a smaller number of dedicated air refuelling tankers for use in other situations where longer range, endurance, or larger fuel offload is necessary beyond which vanilla Y-20 tankers can't provide.
If PLAAF eventually procures, say 200 standard Y-20 transports, then that means potentially 200 airframes for a competent medium weight air refuelling capability. Distributed, available tanking capability means the PLA will be sensible in providing air refuelling probes to all its major fighters, AEW&C, bombers, and other special mission aircraft which may lack it.
The alternative is disheartening; Y-20 annual production will be somewhat limited in the forseeable future, and even once it reaches full swing, most airframes will go to the standard transport role. It will be difficult to squeeze in airframes for a dedicated tanker variant, and will also be at the expense of losing a strategic transporter.
Given the PLAAF's budget, the demands on both transport and tanking aircraft in the forseeable future, and the limitations of even a high rate of annual production in fulfilling both types of airfarmes if roles were not combined, I believe integrating a tanker function ala A400M and KC-390 into the baseline Y-20 is the best option.