Garthanos
Arcadian Knight
Ideas are not copyrightable nor are methods...If they build it as a toolkit that happens to be able to replicate 5E that might work. But just renaming things probably won’t.
If you say things a different way that is not a copyright infringement .
What Does Copyright Protect? (FAQ) | U.S. Copyright Office
Answers to frequently asked questions about what is protected by copyright.

How do I copyright a name, title, slogan, or logo?
Copyright does not protect names, titles, slogans, or short phrases. In some cases, these things may be protected as trademarks. Contact the U.S. Patent & Trademark Office, TrademarkAssistanceCenter@uspto.gov or see Circular 33, for further information. However, copyright protection may be available for logo artwork that contains sufficient authorship. In some circumstances, an artistic logo may also be protected as a trademark.
How do I protect my idea?
Copyright does not protect ideas, concepts, systems, or methods of doing something. You may express your ideas in writing or drawings and claim copyright in your description, but be aware that copyright will not protect the idea itself as revealed in your written or artistic work.