The thing with copywrite is to put yourself in the original game-makers point of view. If someone used the idea of your game. without your permission, how would you feel? I think mentioning them in the credits won't matter at all as you haven't asked their permission to use any of their characters, storylines or names they are not going to be pleased with you mentioning them in the credits. If someone did the same to you I don't think you would say "Oh well they mentioned my name in the credits so it must be alright.". I think you should really think about whether this game is a good idea.