Range and survivability could be advantages for an heavier attack helicopter.The Z-20's engines are supposed to have higher power right? AFAIK the Black Hawk and the Apache use engines of the same family.
I think it is a matter of time until we see an attack helicopter with those engines. They already got most of the propulsion and flight control technology with the Z-20. Just think of the time it took for the US to develop the Cobra from the Huey. They did this really quickly.
It isn't even their first attack helicopter, it will be the third? They are used to doing such conversions already I think. Just look at Z-19.
The major issue, I think, will be to develop new weapon systems that take advantage of the heavier helicopter. Things like rockets are easy enough but missiles not so much.
Maybe a bigger gun and more ammo for better close-air support ? 8 antitank missiles and 2 big rockets pods are enough for most mission anyway.