Huge and hot topic as I can see.
There a lot has been said what could and should be improved (dynamics, hair, bodypaint etc) but so far a proper integration of RS is needed the most in my opinion. I still have to use my old xpresso materials presets and can't switch to a new and sexy node system cause it doesn't support many features that xpresso does. The feeling that RS is just a plugin that tacked on and not a native engine is still there.
And the second is a viewport.
I know it's de très mauvais goût aka a bad manner to compare to Blender but having a proper viewport render like Eevee or U-render is super needed.
The production speed increases as a content shortage so most of the times I don't need to render out a proper render with GI, reflections etc.
Good viewport render like in a Autodesk 3DsMax or Blender would be a huge help.
Thanks,
Anatolii