I think any huge reinvention will be more of the window dressing type at this stage. They know they'll never be the big new thing at anything anymore. They're not going to lead from the front at anything anymore (musically). I think the next overhaul, if there is one, will be subtle musically and will come more in the form of image/tour etc. The album may lean one way or another, show a leaning toward a certain influence over others, but it won't be U2 like you've never heard them, unless it is actually the stripped back 'sound of 4 guys in a room' - which they are yet to actually do.
I think when it comes, it will come when they're good to take a step back and allow themselves to just 'be'. Not be the biggest. Just be. There'll be a point where - hopefully - they'll reach the fork in the road where they realise they can become the Rolling Stones, or they can continue to make new, great, challenging music, and tour wise, realise they can either continue forever with mass stadium shows selling millions of tickets based on their back catalogue and brand (again, see Stones, Rolling) or do what suits the music they are creating.
A leaning towards that more delicate, deeper water, 'quiet' sound of Ground Beneath & Stateless, if it comes about, is to me a sign of that realisation. If Bono is still trying to jump around to more Vertigos in a couple of albums time, padded out with more "please let this be a stadium anthem" Miracle Drugs, they've chosen, or not realised they've chosen, the Rolling Stone path.
I hope there's an album or two of U2 consolidation. Not fairly generic rock/pop with Awesome Classic U2 Moments as they are on now, but truly a realisation of 25+ years of sounds and influences, perhaps with a spin on it that celebrates the band, as I said, the 4 guys in the room thing. After that, a band confident enough in themselves, their past career and catalogue, their position in music history etc to be able to just not really care and just make some great music. Even if they don't sell gazillions. Even if they aren't loaded with potential singles. Even if they aren't chock full of 11 songs you can take on the road. Finally just make some albums free of the pressures of U2 Pty Ltd hanging over their heads.