The_acrobat
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I'd guess arenas just because they will feel they need to do something different than 360, and may not be ready to TOP 360.
But 4 years between albums again is kinda crap.
They could have released SOA in late 2009!
Once they made the decision to NOT release SOA the album, they should have at least done like Coldplay did with VLV and released an EP (Prospekt's March) with 6 new songs. Soon, Winter and Every Breaking Wave could be on there, and a few others.
U2 could certainly do an arena leg in the US, as there are probably cities that would not fill a stadium (although 360 Charlottesville, Raleigh, St. Louis and Norman, OK shocked the crap out of me!) An arena leg in Europe seems pointless, because they could sell out a stadium in pretty much any European city
My guess is that we'll see 2 more tours from U2. The 2014-15 tour which will be a mix of arenas and stadiums, and then one final tour in 2019-2020. Which will be a nice, neat 40 year career. That will be the one that tops 360, or at least attempts to. A 3rd tour in the early-mid 2020s is also possible, with the band in their early-mid 60s.