I know that starting up a private server of an online game is illegal. However, what if I have access to the source files of a specific game, and change/implement the code so that it's a completely new thing? (Like improving graphic quality, fixing bugs, updating game engine, adding new quests/items, etc.) Although it would look very similar in terms of items, terrains, HUDs, the source would be a completely new thing. If that's the case, would it be still illegal (in terms of copyrights/intellectual theft) to publish this game?
I mean the firm that has the copyrights of the game I'm talking about are basically ruining the game because they're careless and negligent of its users. And because of that, many people are playing on private servers instead.