The irony is that U2 *can* take the risk these days. In 1988, U2 really only had the success of JT behind them. Yes, they had some earlier hits, but JT was their only mega-hit album. As a result, when U2 started performing with "the greats", many thought U2 were saying that they too were great (apparently George Harrison was one of those critics).
Now, 16 years later, U2 have indeed proven that they rank amongst the greats. As such, doing a R&H-type of album now wouldn't be considered as "blasphemous" as it was in 1988. It would be like U2 collaborating with their fellow great artists.
Therefore, I would like to see a half-live/half-studio follow-up to the next album, with guests artists.
Of course, given that U2 has already done this (and already explored their roots), there's no real need to do this again. Hence, I highly doubt we'll see it.
Therefore, I would like to see U2 release a series of live shows (either CD or DVD or both), ala Pearl Jam. No, they won't be perfect, but U2 performs so often that I'm sure they can get some very very strong shows out there. So what if Bono happens to be a bit off on a song or Edge hits a wrong note or two or if the audience response isn't quite as expected on a song? If 99% of the concert is great, release the damn thing! People are going to make bootlegs anyway - I'd rather see the $$ go to U2 than to bootleggers. U2 needs to loosen up control a bit for these live recordings. Maybe only die-hard fans will buy them, but when die-hards bought Pearl Jam's releases, that gave Pearl Jam five albums in the Top 200! So it shows fans are willing to buy live material and make it a success.