What if someone were to make a new game that can only be installed if you own a copy of "another game". This game itself doesn't modify any files or use any DLL but only textures and models. The game's dll's and its extra resources would sit in a separate folder.
I don't think it can even be considered a mod, but would something like that be legal, considering that this project would aim for profit (whether directly from purchase or indirectly from micro transactions)?