Will the phone have a LAN-/Ethernet-Adapter and an option to get a network connection over USB from the PC, so that users don´t have to use WiFi?
I can not remember that there would have ever been any discussion that there would be an adaptor included, so no. But the USB-C port is fully functioning and it should be possible for you get one yourself. I would suggest using a dock that also charges the phone at the same time - as well as gives you other ports, including monitor and peripherals, if using as a desktop (see threads here, here and here for example).
it could have had one … it’s plenty “brick”
You can use USB networking already when connecting to a PC, using the phone as a gadget.
Using an adapter with the phone in host mode is something that might already work, but I haven’t tried it. See @JR-Fi’s reply for links to relevant threads.