Given that I hate online play this would mean that my current games would have to suddenly become apocalptic, more like the GTA example than any others.
And I think it would be awesome beyond words.
There's something I find fascinatingly morbidly beautiful about games like Fallout 3, or [PROTOTYPE], in the way they portray the familiar as changed. (Not that I would recognise Washington DC or New York that well being British, but even for me the cultural osmosis of the White House gave me an interesting experience visiting it in Fallout 3, and a similar effect with Times Square in [PROTOTYPE])
I can imagine running around Saints Row, or the Mass Effect universe, or Hogwarts, and everything was busted all to hell, and I would think it was the coolest thing I had ever played. A lot of my earliest gaming experiences instilled in me a belief that the apocalypse is the only fitting conclusion to your final battles against evil. Final Fantasy uses it a lot, Mass Effect seems to be leaning this way, HP ends with Hogwarts in near ruins, Lef5 4 Dead, Halo, Batman, even Tomb Raider to an extent ended this way, with you fighting in a near literal version of Hell.
So for me, I like to think that the apocalypse is always waiting just round the corner, the final trick for the villain to pull, the final fight I have to go through, and it's only right that it's the biggest challenge of all.
To have an entire game where Lara Croft explored a desolate and lonely Britain ravaged by the thralls and the shadow monster from the latest three games would be so incredibly powerful to me, and I would love to see that image realised someday.