I suggest keeping the focus on behavior. Things like treating others with respect, no swearing, not quitting early when you are the host, not anchoring to the red zone, no spam-blobs .... things like these make for an enjoyable game between the involved players. In a Code of Honor I would stay away from things like trying to control the composition of armies, banning units or factions, etc. These are really game balance issues. They are important to tackle but I think should be done outside the realm of an honor code.