U2 is one of the most creative and challenging bands within the sphere of pop music. Their unceasing efforts to improve themselves and expose their audience to new musical styles are equalled only by the Beatles' efforts, in my not-so-humble opinion.
However, just because they happen to have a varied discography does not mean they are not susceptible to patterns. If one examines their artistic progression, a pattern definitely emerges. One which may, in fact, provide clues to the type of music we can expect from them in the future.
Here is the list of their studio albums, excluding EPs and side projects:
POST-PUNK ERA
Defined by post-Clash instrumentation ("punk rock with a delay pedal") and lyrics reminiscent of The Who.
Boy (1980)
October (1981)
War (1983)
AMERICANA ERA
Defined by moody atmospherics and lyrics obsessed with American icons, foreign policy, and the American Dream.
The Unforgettable Fire (1985)
The Joshua Tree (1987)
Rattle And Hum (1989)
CYNICAL GLAM-ROCK ERA
Defined by flirtation with industrial guitar textures, dance rhythms, and self-consciously ironic lyrics that seem trashy and throwaway on first listen.
Achtung Baby (1991)
Zooropa (1993)
Pop (1997)
NEOCLASSICAL ERA
Defined by a return to the sounds and atmospherics of the Americana era, with subtle rhythmic touches culled from their Cynical Glam-Rock phase.
Lyrics are simple, direct, and to the point.
All That You Can't Leave Behind (2000)
How To Dismantle An Atomic Bomb (2004)
???
Once you see their body of work laid out like this, a pattern just kind of jumps out at you. U2 work in triplets. They spend two albums working on a specific genre of music and making it their own, then take that style to its extreme for the third album before departing entirely from the genre.
Look at the Americana Era. The first album in that era, The Unforgettable Fire, is a drastic and marked departure from the album that preceded it, War. In fact, I wouldn't be remiss in calling it U2's 80's Achtung Baby, since it even notoriously upset Kurt Loder in his Rolling Stone review of the album. Then they continued that musical vein with The Joshua Tree, only to plunge into full-on roots rock with Rattle And Hum's studio tracks.
This is no isolated phenomenon. U2 do this time and again, no matter what musical vein they happen to be mining.
So, with that in mind, what can we expect from the follow-up to How To Dismantle An Atomic Bomb? As U2 are in a renaissance period of sorts, stripping back their 90's aspirations in favor of directness and earnestness, it follows that the next album will be even more stripped down, almost to the point of garage rock. The lyrics will probably be the most overtly religious lyrics they've written since October, if "Lead Me In The Way I Should Go" and "Mercy" (two outtakes from the How To Dismantle An Atomic Bomb sessions that didn't make the cut) are any indication. Maybe it'll be a garage-rock praise and worship album, but I highly doubt it.
The above, of course, is just for fun. I myself highly doubt that any of the preceding conjecture will actually come true. But this kind of thing is endlessly entertaining, at least for me, especially since I can't stop myself from making it.