I'd REALLY love to see Nigel Godrich come in and give it a go, personally (Radiohead, Beck, etc.). He's far from perfect (he also works with Travis...so there you go...ugh), but he regularly helps bands to push boundaries and to maintain their essence, at the same time; the album would be very exciting, I think...I already hear the over-emotional rush of U2 mixed with the spacey vagueness and parity of Joy Division. I would love it.
I never want Eno or Lanois to be at the helm of any U2 albums EVER, so they're out for me. I would like to see Flood take up the reigns for an entire album again, though--it may not be perfect, but I'm sure it'd be exciting as all heck.
I could also go for the Dust Brothers--they've proven that they can rock do just about anything, and we all know that Bono's been all about the "MMMbop," as of late.
Seriously, though...they're very capable and hyper-talented and they could easily help the band to push itself in new, relevant, powerful directions. They've done some of the techno stuff that U2 has flirted with, and they've also dabbled in ambient-esque soundscapes, so I think such a pairing could really shake things up and make it really fun both for us as listeners and for the band as artists.