ok, putting on flame retardant suit of lights... (see what i did there?)
if bono steps back from the campaigning and concentrates more on his craft, his lyric writing... then yes, i believe it is possible that u2 could create an album that is greater than anything else they have ever done. as u2 as a band have been getting tighter and more mature, the thing that hasn't come along for the ride has been bono's lyrics... which have been brilliant at times and absolutely dreadful and cheesey at times. when the man is being pulled in so many directions, it's hard to really blame him... but as great as the work that he does is, i'd love for him to step back, pick and chose his spots, and let somebody else run with the torch for a while.
all that being said, despite the fact that i do believe that u2 can, creatively, still come up with their best work, i highly doubt that most of the masses will hold that work to be greater than the joshua tree or achtung baby, simply because that's how things seem to work. except for rolling stone... they'll slurp anything u2 does.