I'd sort of rather they just made a new album and toured.. as boring and mundane as it is. But if they did decide to did, it would certainly be interesting.
I kind of think that they have already done their 'experimental' kinda thing, with Passengers, I dont know about this rock opera thing- never been a fan of it myself, I am a huge fan and hated what they did, kind of thought it was Pete Townshned indulging himself too much, so I would like to see U2 just keep doing some cool music
I think the rock opera thing would be pretentious shit. Although I did see "Tommy" twice on Broadway and was thoroughly fantastic.
The orchestra thing ala Metallica I think could work - but could come off as pretentious.. U2 often walk a fine line sometimes - and the press is always waiting to skewer them if pretentiousness arises.