[jeremycw]

I am not a lawyer and please correct me if I'm wrong but as I understand it rules and mechanics are not copyrightable. What is copyrightable is the flavor text and specific compilation of the rule set into the "Rule Book". The rule book becomes a copyrightable work. This means that your are within your legal right to make a game called "Dragons & Daggers" where all the combat mechanics, rules, etc are identical to Dungeons & Dragons as long as the "glue" text and flavour text is original content. It seems to me like a community fork licensed under CC that plays exactly the same is not impossible.

[advisedwang]

Spells use text to explain their mechanics, and that text is copyrightable. You'd likely need to re-write the spell lists from scratch.