Eh, I think if U2 wanted to play stadiums, their people will find a way to transition the current show to stadiums. I just can't see U2 doing another arena leg in Europe. One reason is because of demand, another reason is because there really aren't many good arenas in Europe left for them to play. Also, South America doesn't have many good arenas, and stadiums are the only option there.
On the other hand, I can't really see U2 doing stadiums in the US/Canada, due to LACK OF DEMAND. I think we were all a little surprised that the NA shows didn't sell as well as we all thought they would, and I'm sure the band / their hands-off manager / livenation were surprised as well. Australia is about 50/50 on arenas vs. stadiums. Most artists now are playing shows in Asia; Dubai, India, Thailand, Singapore, Philippines, Hong Kong, Korea, not to mention Japan. This would likely have to be in arenas, if U2 were to do this. Since U2 played the Tibetan Freedom concert in 1997, I doubt China would let them in.