The only "major" bands I see that aren't available as standard or as DLC are probably Metallica, AC/DC and i'd like to see Soundgarden or official Tool and Slipknot songs. As always with these things, it isn't down to Ubisoft to just "create" these tracks, there are countless licensing issues. The big thing is the likes of Metallica and AC/DC are huge bands and probably demand a bit more in terms of royalties. It could also be that these two bands in particular are big enough that they could hold a release all of their own (as they both did with Guitar Hero and Rockband respectively). Maybe they are waiting to see how sales of Rocksmith do and are waiting to release their own special editions??