I'm planning to make a GPLv3 or AGPLv3 library to allow the library user to interact with the multiplayer portion of a non-free, closed-source, proprietary game.
The game's EULA explicitly allows the players to customize the game, create and share mods but has some protective restrictions to prevent the game from being pirated, used commercially, or with intentions to make money out of it. It also distributes the game server binaries publically so that people can host their own dedicated servers.
This is what confuses me, I know that I can't use the game's code or link to it and I'm not willing to, the library code will be written from scratch, but can I make a client/server library with a compatible protocol so it can be used to host customized game servers for example? Do I have to add an addendum to the licenses or can I use the plain GPLv3/AGPLv3 licenses as they are?
There are some community-made server Softwares of this game, but I don't know how legal they are. I want to make this project completely legal since the beginning.