I think are perspectives are too easily twisted by the thorough change in the musical landscape that came with the internet, simply because the internet has brought musical history, like everything else, to the ears of a much greater group of people. In short, I think people in general are becoming more and more aware of the influences of their favourite bands, which makes them less inclined to label something as "like nothing else" and with the internet they can easily buy or download (legally or illegally) tons of stuff that previously they would've never even heard of. I think time will change what we think of musical history, and it's already happening to some extent.
Also, with more people getting in touch with older music, the previous sales will mean less and less, as sales lists are I think losing their effect on what people are buying.
I think the whole term of "legendary" will for a large part lose it's meaning, and good riddance with that. There's just so much mediahype going on everywhere, I think younger generations are not that easily duped into calling something "the greatest thing ever", as if such a thing ever existed.
Also, I think some bands might very well lose their legendary status, becoming more and more obscure. I think Led Zeppelin is to some extent going that way, I don't know many younger people that listen to Led Zeppelin.
Some bands will rise to legends somewhat surprisingly, like Queen, which was recently voted as the #1 British band ever (yes, better than the Beatles). True, Freddie Mercury's death had a big impact on their sales, but a lot of their current status really goes to their two well-made collection albums, Greatest Hits I & II, which introduced a whole new generation to string of amazingly catchy and creative tunes, which were previously spread out over many mostly quite uneven albums, and those collections also had the audacity of actually including songs that were not hits previously. Seriously, it's really weird that so many of us know each and every word to a freaky song from a crappy movie, buried in a horrible album, and yet millions know and love Flash Gordon's Theme.
So looking at all that, what bands could become legendary? I think the primary issue is "are they _instantly_ recognizable". From the bands mentioned here, Modest Mouse and Tool are close to that category, but Tool suffers greatly from the fact that their really kind of a one trick pony, musically speaking. Sure, they do a lot with that one trick, but I for example still can't attach a name to any of their songs, even though I know I've heard most of them dozens of times. They really just sound alike, if you're not into it, and that I think will forever stop them from becoming anything but a cult legend. (Also in my opinion they suck as live band, but don't kill me for that okay.)
Modest Mouse on the other hand is just way too clever. To become a legend, millions of drunks have to be able think they can sing your songs and think they know what they are about.
Metallica I think is somewhat underappreciated due to their decade-and-a-halfs decline, since really the Black Album is in my opinion the single most influential album of at least two decades. After that, every year tons of albums are put out that essentially try to copy that exact sound, and it almost singlehandedly pushed metal into true mainstream and at the same time redefined metal (in many peoples opinion for the worst.), much like they did with Master of Puppets and Justice for All. And you can still tell that album apart.
Nevermind on the other hand is one of the greatest rock albums ever, but really I think it's just really really great songs, not really anything that wasn't done before. Kinda like the Beatles. Amazing tunes, but most of their greatest hits aren't exactly groundbreaking.
I think Faith No More will have at some point experience a resurgance in mainstream accolades, as The Real Thing and Angel Dust are still quite remarkable albums and the band really pushed forward what modern rock could be. Unfortunately lot of the bands that followed them are filed under the sad chapter in rock history called NuMetal.
I think Chili Peppers are up there too. Great songs, and they were also a big part of putting funk back into rock.
Once all this pedophilia-thing is forgotten, and eventually it will (especially since the guy was proven innocent, although many people have a hard time accepting it, propably because it's just not as interesting), Michael Jackson will be hailed as one of the greats, undoubtedly.
After that I'll go wild: to achieve true legendary status a band essentially has to be on the forefront of breaking a new style into the mainstream, so I think System of a Down has good chance of becoming a legend. For now they are way too extreme for most people, but they are already hugely popular, and in time their chaotic style (which IMO degraded slowly into a total crapfest after the first album) could become mainstream. Seems incredible now, but hey, listen to Metallica's Kill'Em All and imagine that those guys are essentially legends now
Madonna deserves to be a legend, for extramusical things if nothing else. In pop, that stuff matters.
I think Britney Spears might easily achieve a great status alongside pop starlets like Donna Summer and Gloria Gaynor. In time, no-one will care about her personal life and her Lolita-marketing, and what they will have is a few really good pop-songs that a full generation knows by heart, like it or not.
Some eighties greats are also due for a revival, so what the hell, maybe pop will strike back and Duran Duran will be elevated to greatness. Maybe Depeche Mode will come up with another massive hit album, which might be quite enough to turn them from classics into legends.
But really the next big thing will be an artist doing something that right now doesn't look like they could ever make the mainstream, and it's impossible to say who it'll be. Maybe someone from the scandinavion modern folk scene will write songs in english and turn something most people have never heard of into mainstream. That sound deserves to be recognized.
But unfortunately due to the language barriers of the world, most artists that would be worthy of legendary status will never achieve it, simply because they will remain the national treasures of their respective countries. From Finland I should name an artist known simply as Risto (nothing to do with me, even though that's my name also) as someone who has made one of the strongest impressions on me ever, especially as a live performer, and I count the artists I've seen with three digits.
Rammstein has to some extent broken the language barrier, and I think they should definitely be hailed as an industrial/metal legend already. But bah, so many people do not value true fun.
White Stripes I think are vastly overrated, IMO they just recycle old stuff in a crappy way, and will eventually be caught from it. Flaming Lips are quite interesting, but I really think they're more of a live show than a great musical groundbreaker.
...I'll rather think about this stuff than the "game" that's going on...
