A bunch is in D2 already, literally at least half of the Music of the Spheres is in in game whether it be the elements or actual excerpts that I know of just from the top of my head. All of the Awoken music and some Taken King music is in the game along with some vanilla stuff. I'd say at least 20 total tracks with quite a few D2 tracks having strong elements of D1 stuff. I should also mention the Music of the Spheres version of stuff was never meant to be in game in the first place. Its a special standalone composition, this is most evident in the fact in D1 and D2 these tracks when they do appear are classified as an excerpt. It just has the key part and is slightly different to fit the game, especially when actually having it play during a mission or cutscene. What you're asking for already exists, we dont need everything and there's probably a lot more that are going to get brought back when future DLCs drop anyways.