Right. The idea here isn't to create something that directly in direct conflict with existing trademarks, but to allow people the opportunity to explore the mechanics of titles that may have inspired them to get into game development. I am not a lawyer, so this is not legal advice, but as far as I'm aware legal troubles for people mostly start when someone begins to charge money for their product, using IP that is directly owned by another company. It would be my advice that anyone who hopes to make it big or make money off of their product, avoid the use of assets and trademarks that are not freely available to the public, or owned by existing entities.