This already exists (it’s not a VPN, it’s more of a proxy); it’s called Relay. It does have the caveat that it’s bandwidth limited (2 Mb/s for Plex Pass subscribers, 1 Mb/s for others). It’s not a perfect solution for everyone, but it does allow remote access where it otherwise might not be possible.
This would be great, and there’s an existing feature suggestion which covers it. There’s a workaround for using IPv6 with a custom server access URL in that thread: