Copyright and Trademark Issues for Expansion Pack for Game

Dear Rich: I read with interest your post "Do Not Pass Go: How to create non-infringing board games" and I have a related question regarding expanding existing games. I have created an expansion for a popular board game. I do not use any of the original game trademarks in my naming or design, and I don't replicate any of the original game's physical, visual, or textual elements. Can I legally market my game? You may be legally correct in what you're doing -- that is, you're not infringing any copyrights, trademarks, or patents of a famous board game -- but if the board game company perceives your work as chipping away at their potential revenue or trading off their famous trademarks they may sue first and ask questions later. Why is that?
Inside the mind of a board game executive. Perhaps it would help to imagine what happens if you put yourself into the mind of someone at the legal department of a big toy and game company ... for example, Mattel. Fade in on your office. You sit behind your desk, sipping a chai latte and looking at the framed posters of Chatty Cathy, Barbie, Tickle Me Elmo and several Hot Wheels vehicles while you sneak time to work on your screenplay. You quickly switch screens as your boss comes into your office to show you a print-out from a website. It's an expansion pack product for a Mattel board game. Your boss -- pleasant enough in the office, but a tough cookie in licensing negotiations -- says, "Memo Me Elmo" which means you better come up with a good legal theory for going after these guys because after all, this is business, and Mattel wants to earn any and all revenue resulting from the success of its games.
Your memo. You know there are a few legal cases related to these issues and you know that Mattel will have a legitimate claim against the expansion pack maker only if: (1) the game company can demonstrate that consumers are likely to believe that the expansion pack is endorsed by, affiliated with, or commercially connected with the game; (2) the game company can demonstrate that the expansion borrows enough copyrightable elements from the board game such that it is a derivative under copyright law; (3) the expansion pack somehow dilutes or tarnishes the trademarks of the famous board game. You study the advertising for the expansion pack at the website and look for references to your famous game's trademarks and then you buy a copy to examine the expansion pack in detail. A few weeks later, your boss directs you to send out a cease and desist letter. Fade out on legal counsel.
Takeaways.  Even if the law is on your side and your expansion packs may be legitimate products (subscription required), turf-protection by game companies may hinder or slow down your ability to get to market. We hate to stifle your creativity but you'll notice that the case law we cited was between well-heeled businesses able to take each other on. Otherwise, you'll be flipping through Yelp looking for an attorney.