As long as the mod is treated as a mod, there is no problem.
Even if it recreates some parts of an IP, it would still be very hard to sue as it's not strictly a carbon copy, actually, it's not even a game itself, it's a customization of a game. I'm not an IP lawyer, but in my view it could be described as a fair use case very easily.
Usually IP lawsuits happen when there is money involved, the similarity is too high (your product is trying to pose as my product) or damage to a brand by association with something their owners do not want to be associated with.
We could cite Shadowrun Returns, there are mod projects to recreate the games from SNES and Genesis using the engine of the new game, there are also projects that recreate some of the runs that only exist on the Pen and Paper game. 0 lawsuits happened so far, heck, the developers even PLAYED some of these mods in a live stream.
As long as people keep their mods as mods, lawsuits should never happen. And if they do, nothing serious is likely to happen.
This is actually one of the few positive things about the law being something that evolves so slowly.
