I don't think this would be of interest for anyone surrounding this project. Requiring you to buy the original game keeps the publisher happy (to a certain level) and also gives you an argument for legality regarding the non-graphical content which might also be copyright protected.
But then you need to replace dialogues and storyline as well ? Also items/spells/mechanics can't be a straight clone ? So you would end up with a completely different game ?
Stat blocks aren't protected by copyright. Most game mechanics aren't patentable in the first place and Diablo patents would have expired.
Names fall under trademark law and even names that they've bothered to apply for (let alone all those rejected for various reasons) aren't difficult to change.
Story is a different question. The core story is incredible thin and derivative. Creating a new story wouldn't be very hard. That said, most players today would probably be much more interested in multiplayer anyway.
I'm thinking more along the lines of names + stats, and game rules in general, if that wasn't copyrightable I'd imagine nobody would have to license D&D to create a video game based on it.
The code is not the problem, nobody says that. But everything which was created as part of the Diablo universe can potentially be subject to copyright law.
