Why not go back to the Wasteland technique. Can you imagine a modern game where the gameworld gets copied to a backup and THEN created on that backup so that if you destroyed a building, it could stay destroyed if you went back to it!
With DVD technology, we could have part of the gameworld fully inhabited, but fully destructible.
Ie: Imagine if in GTA 4 there was nothing over the bridges, that the boroughs would come on separate add-on's but would be just a full of quests, etc as the first, eventually you would have the whole of New York, like you do with GTA IV, but with 5 times the content. Now sell the game with a backup CD, load from that backup, and now you can have a destructible world. Maybe you could even have a texture disk that you could load that would have half re-built building textures making like the gameworld was 'repairing' itself.
In other words have a core game and then keep adding to it so you could have a Daggerfall sized world full of quests, NPC's destructible landscapes and building and a whole lot more. In other words, use modern disk and multitasking technology and give us a Fallout 3 world that evolves for years, with wars rising and falling, towns being destroyed and rebuilt, NPC's permanently dying and their gravestone created in the graveyard! The idea's are endless, but it involves thinking outside the box a little.