The using of assets thing is going to be the major problem. Modders use one another's assets and textures and models as a matter of course. They use free versions or "not allowed for commercial use" versions of third-party software to create art assets and models.
Or...they unintentionally copy other mods. Bubba gets the cool idea to sharpen textures to a higher res by applying a sharpening filter, while employing a methodology to save on VRAM. Chuck, totally independently, notices the same thing, and does the same process. Both guys release their mods, and then both guys accuse one another of stealing.
There are mods within some game communities that seem to be an easy thing to check for this kind of stuff. CSGO gun skins, for instance. But how would you like to be the person at Steam who does quality control for the Enderal mod, looking for any borrowed assets?