So far there were no plans to port ZServer to Python 3. I'm neither the most async guy around so I cannot really comment on async-capabilities in a WSGI-setup.
My concern is currently the best-practice of deploying Plone and waitress seems fine and I also like the fact that there are options like gunicorn and uwsgi that we can use. ZServer's job as a http-Server seems to have been taken over by something well maintained and flexible.
Do you plan to use ZServer as a http-Server for the whole of Plone or are you only interested in the asyc features? Do you think it would be generally better for us to use ZServer over one of the existing WSGI-Servers?
You should probably talk to @tschorr since he tested and wrote most of the wsgi-setup in plone.reciple.zope2instance.