Varnish 5 is the first version to support HTTP/2, minus TLS termination. The Varnish team advises using their terminator, hitch. This sounds like a saner stack than combining Nginx and Varnish like we do now, and the HTTP/2 protocol would probably be better handled with two pieces of software that were designed to work with each other (technically, Varnish would be in charge of what matters in the protocol: assets prioritization, interleaving, etc.).
Unlike the upgrade to Varnish 4, this one looks a lot simpler, with few changes required to VCL: https://varnish-cache.org/docs/5.0/whats-new/upgrading-5.0.html#whatsnew-upgrading-5-0