There's going to be a music dungeon that you can get stuff from, including some of the Milky Way instruments (from dailies I think? I can't remember). There's also gonna be new quests, one of which the GMs said will give the Milky Way Lute, if I remember correctly. The composing interface will be updated with a keyboard and a couple of other things, and there will now be a board for sharing MMLs in-game. We'll also be getting some hot-time events, although I don't know what we will get from them. HOWEVER, I think one of them, iirc, is the Music Q outfit the GMs were wearing in the stream. A new action we'll be getting from the update is the Freestyle Jam, which is kinda like DDR but with instruments. There's also a...I can't remember the name for it, but there's a jam mode where it should cancel out all other sound (or at least other instruments) so you can play your jam without interference. I also heard (not from the stream, but forums) that the instrument midis and how some instruments were being handled were getting updated, but I'm not 100% sure how much so.
Most of this is scattered bits I remember off the top of my head from the stream and forums, but we'll see what we'll get specifically come Thursday.