Both OpenTTD and FreeDoom have identical gameplay to the originals but different assets. Also, the level layouts are different in both games as the scenarios are copyrighted, not the concept of the game themselves.

If anyone wrote custom cities, textures, scripts and so on with the OpenMW engine you are totally free to do so even if the result looks eerily similar to Morrowing but not being the same game at all, if any sharing a fantasy RPG setting and that's it.

Ditto with OpenArena being a total clone of the Quake3 Arena concept but with different levels and assets, and virtually it's the same game at a 99%. You can totally sell OpenArena or any new game reusing these assets if you comply with both the GPL and the CC license from the media.

Dave Gnukem it's an obvious Duke Nukem (pre-3D) clone and even if it can't play the original game, it can be trivially adapted to reuse the original textures and level sets in order to get a very close gameplay to the original. And yet no one sued them.