Thank you for the replies!
@Posh Indie the models are also released and some people made snow and desert reskins. Their lights and shadow direction already clash a bit with the rest of my game but i'm not sure which style to keep(e.g. is better) and which to re-render.
@Jabbers Could you elaborate on the music?
As long as you can extend the asset pack you should be fine. I think you should be less worried about public domain assets and more worried about people drawing a parallel to Diablo. Resembling Diablo is not, in itself, a bad thing. The problem will be more of differentiation at this point.
You probably don't want to be in direct competition with an established title made by a company with a fanbase as large as Blizzard's, so you will want to find a way to shake the image of competition. There also lies a problem with differentiating from Diablo, and that is the people who will expect it to be like Diablo based on appearances alone. My advice is to find a way to let people know it is different from Diablo, and make sure they know how it is different from Diablo before they purchase (You do not want the wrath of Steam's refund policies).
If you are not afraid of a possible backlash from Blizzard's fanbase, then you can put yourself in Diablo's realm directly. They will either appreciate it for what it is or raise the torches and pitchforks (Diablo 2 had gotten a relatively recent update, so Diablo 2 may not have actually run its course even with Diablo 3 being released for some time now.)
As far as public domain music is concerned, I would say "Be very cautious". My reason for this is better explained through a question, "How many times has a song come on the radio and you recognized it within the first few notes?" Music seems to be very easily recognized and I would say it is less acceptable to reuse musical assets than it is to reuse graphical assets. Graphical assets are easier to mask when repurposed whereas musical assets are harder to mask (There is so much to a musical score that flat out gives it away. The changes you would need to make would be great enough and impactful enough that you might as well just make your own music to begin with).
A disclaimer, though. The games industry is fickle. Predicting the industry is hard because you are analyzing a huge and diverse group of people. Extrapolating information from data guarantees nothing because the industry can sway at any time and in very drastic ways (Also your data would need to be large enough to accurately represent everyone). Marketing to your target audience is very important since it reduces the "variables" in prediction. Most definitely focus on that (Does not mean you definitely have to pay money to do so), because games that succeed without marketing hit the market in the sweet spot and at the right time to leverage word of mouth (Great and cheap tactic if you can hit the market just right). In short, "Your game can succeed just because, and it can fail just because, as well."
The fun is actually releasing, doing what you can to improve the odds, and seeing what happens.