Spoofs are not ilegal. In fact I think the law says you can spoof anything you want. I know for a fact that celerbies are not allowed to sue someone for making spoofs based off them. As long as its clearly a joke I don't think they can do anything. To be safe do not copy anything exalty. Make everything a "likeness" of the object and give it a parody name. Also don't mention games by name because then they can get you for product placment charges. And most of all do not copy the games companies logo. They can defanitly get you for that. You can make fun of other games plot but don't have your charactor "follow" the plot. Basicly don't copy the game itself.
Why would the story line stop if you play as good player? That makes very little sense. That's just forcing people to play as a bad character. And why would there be more enemies if you bad? Are there no bad enemies in the game or something? There should be differences in play style, but they should be subtle