You don't need copyrights for a mod.
Don't listen to dusty.
Co-op online play is not a mod. I'm trying to bash into your head he does not nor will not support these kinds of games, nor does he want them. And you are prepared to go through the intense scripting, spriting, story, etc? I'm afraid it's not that simple. CS's scripts are embedded into the executable. And the scripting format is custom and most likely will not allow online play. Are you also prepared to pay for a master server, like Blockland?