IANAL, but generally I don't think pure game mechanics are copyrightable/patentable. This is why people can clone games with impunity, using the same mechanics, control scheme, etc. but putting a different name and different art, sound effects, etc. on it. So if you want to make a game with similar "design ideas" (I presume that means mechanics) to an existing game/mod, I think you're fine as long as you don't copy their name, art assets, and so on.