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.