Another thing is that the Mi-17 is not exactly in the same class as Z-20 or blackhawk, it is bigger, less maneuverable, and requires larger landing surfaces. Mi-17 is a more cost-effective solution to transport lots of stuff under more tolerable conditions (good landing area, normal altitude, not in extreme weather, etc.), but it may not fit the tactical needs in harsher environments. I think the PLA will continue to need both an economical workhorse like the Mi-17 and a high performance "front-line" medium lifter like the Z-20. In the long run they may use Z-18 to replace Mi-17 if they manage to lower its cost, or they may develop a twin engine 13-ton chopper, or they may even buy the Mi-38. The Z-20 fills a role that the PLA didn't have a perfect fit before, but I don't think it will work as a complete replacement for the Mi-17.
closest American equivalent to the Hip is probably the CH46 Seaknight, for the Europeans Merlin or Puma.
The Blackhawk is more of a intermediate bird sitting between a lightweight like the H-6 little bird and the medium S92 SuperHawk.
Z20 would likely similarly sit between Z9 and Z8. Max troop load is maybe a dozen not counting crew that would of they follow the Blackhawk standard pilot and copilot plus one or two gunners.
If Z20 is as capable as Blackhawk then the question is just how closely they will use the Blackhawk as the roadmap. Meaning roles to include transport, ASW, CSAR, medevac, special ops and battle hawk a special operations version packing a turreted chain gun and external arms. If they do follow close perhaps even a stealth version. But that's down the road some distance.
now this still leaves a capacity gap in heavy lift chopper class. Option one would be to procure a a true heavyweight like Mi26 option two procure a heavier medium lift and make due much like Chinook Which sits between hip and CH53.
Given the tilt rotor program the PLA has admitted to I suspect they may be looking to that to operate as there future heavyweight. The quad configuration seems to me suited to a platform roughly equivalent to the current American Chinook or maybe CH53E super stallion. I am not sure it could as we have seen though go near Mi26 class.