Death - Holy shit, just when Chuck was becoming "progressive"...I just wish I could have seen them live and that they were still doing albums and...god they would still be kickass, I'm sure of it
Arcturus...and it doesn't matter the vocalist, Garm would be great to have seen them with and I think Vortex really shined with them (even more than his stuff with Borknagar).
Windir is a big one too, what a magical band, I wish I could've been at that Milwaukee show, then, because they fucking rule.
Pantera - I'm not a big Pantera fan, but I still wish Dimebag were still around, y'know?
Pink Floyd - If I could see the lineup from The Wall or before, it'd be magical.
Emperor - I'd love it if they were a band that still put out new content and did significant tours
Sepultura with both Cavaleras back (and them actually playing thrash again, goddammit)
Cryptopsy - a tongue in cheek answer, but shit, everything's already been said about the "new" Cryptopsy and I don't need to repeat it here
Green Carnation - I did get to see them live before they call it quit (on their first and only US tour, which was not a good one for them, attendance wise). They are incredible in just about every way, and I will miss them.
Gorgoroth - I wish that this split personality thing would end and they could become one cohesive group, I'm fucking confused right now of what to actually call Gorgoroth!
Spice Girls - 'nuff said. Spice up your fucking life!