I'd love to see something like post-Empire Trantor from the Foundation series or that medieval, post-apocalyptic, Canticle-for-Leibowitz-style Earth from Babylon 5's The Deconstruction of Fallen Stars, complete with a secret group working from the shadows to speed up their recovery so that they could rejoin the galactic community as soon as possible. I think it could be interesting to see a Star Trek take on this, especially how the Prime Directive would come into the picture.
At any rate, I'd personally love to see an Earth that's not among the six remaining members of the Federation, because that would be an interesting dynamic, even without reducing the planet to rubble. Maybe an Earth that's still more or less as utopian as it was in the 24th century, but its people are hopelessly out of touch with the fact that they're no longer relevant to the rest of the galaxy.