Contests/questions based on external games

Question

There are a lot of interesting ideas out there, in the form of existing games.

There is also a strict rule that anything posted on SE must be licensed under cc by-sa 3.0 with attribution required.

Where is the line between unacceptable posting of someone else's copyrighted game idea as a question, and acceptable use of a copyrighted game as an inspiration? Separating your key ideas into short, separate answers will allow the votes to show community consensus.

Discussion in chat occasionally turns towards existing board games or computer games that might make a good code golf or king of the hill contest, and I'd like to see some clear guidance on what we need to bear in mind.

Answer 1

According to this GameDev.SE post gameplay is not copyrightable. Hence, any game mechanic can be turned into a challenge. The legal issues which you need to worry about are:

• The textual expression of the rules does qualify for copyright protection, so you should explain them in your own words.
• Don't reuse any assets (which is unlikely on PPCG anyway)
• Maybe refrain from using trademarked names, like specific Pokémons.
• In some cases the game mechanics may be patented.

Further sources:

• U.S. Copyright Office
• Article on NYTimes.com