It should fall under the exception of Fair Use in US Copyright law, and determination of whether it is Fair Use or not would only really matter if Blizzard decides to take some sort of legal action. Until such a time occurs (if at all), nothing is stopping the OP from creating a mod.
Now you are being silly for the sake of being silly, fair usage doesn't apply here. Go read one what fair usage is before I start copying and pasting fair usage articles.
Making money has very little to do with fair usage, the biggest mistake about fair usage is people think that fair usage means you are not making money, a lot lawyers will tell you that this is simple wrong.
Now if we were to remove money from the question and we talk about Another big Game developer copying diablo 2 it will be an outright illegal activity hence why no one dare does it. They make similar games but not a straight on copy.
