It is my opinion that Andy is correct that music moves in 20-year cycles and that right now we're in the so-called "80's Metal phase" with bad emo hair-swept-over-one-eye-with-dyed-chunks style taking over from the big, teased aquanet classic.
However, I think that strangely, there WON'T be a "Next Big Thing". A "Next Big Thing" (henceforth to be referred to as the acronym "NBT") requires a more focused music listener than the vast share of today's audience can provide. I don't think that there is enough attention being divulged on any one band, or better yet, group of bands to set off a chain reaction. I think that illegal downloading has made it difficult enough to find footing as a band financially because between myspace and bit torrents, we've basically invited everyone in for free cookies and beer, got stampeded and either were deemed unpopular enough to be ignored, or they cased the joint, decided they liked our TV and broke in when we were out later that night and stole it from us.
Furthermore, thanks to genre-crossing bands, everyone listens to everythign and there is no target audience anymore. When a product has no target audience, it dies. Business school 101.
However, music isn't like any other product. If you ask someone what kind of beer they like and they may say "I don't drink beer, I drink wine" or "I don't drink". Or something. Not many people seem to have noticed that when they attempt to strike up a conversation with someone else, they always say "what kind of music do you like?" not "Do you like music?". I have never run into anyone who said they flat-out didn't like music. My theory: Music is like sex. We all want it.
Thanks to the DAW and the internet (or even more broadly, computers) we can all make music now and no longer need to pay anyone to make or listen to it. My experience with human beings also dictates that once we have something we start taking it for granted but the moment it is taken away we are infuriated.
In other words and to bring it all home, my theory is that the music industry will never be the same again and that due to all the aforementioned information, that a NBT is a lot more unlikely to occur from this point on. In terms of acts like say, Nirvana anyways. But, in terms of genre cycles, I agree, with Mr. Sneap; there will always be popular movements. Next stop, grunge, baby. Once more into the breach...