According to a new report from Car and Driver, the next A8 is targeted for 2016 or 2017. Part of the next design will most likely employ some form of "aluminum hybrid" structure, meaning a combination of aluminum and high strength steel as well as other composite materials (the current A8 rides on an aluminum spaceframe). Porsche is working on new modular architecture to underpin the next Panamera as well as the next round of Bentley sedans, and according to the report, the A8's successor is likely to be based on this architecture as well.
Though a plan for the next A8 has not been finalized, there is good reason to believe this transition to the Porsche-developed platform may take place. Wolfgang Durheimer, the new head of research and development at Audi, came over from his previous post as Porsche's R&D chief, so he is already familiar with the Porsche architecture. This shift would mean that the next A8 would be based on a rear-wheel drive platform. Even though all U.S. A8 models rout power through Quattro all-wheel drive, front-wheel drive models are available elsewhere in the world, as the chassis is FWD-based. C/D notes that this move would make the A8 the first RWD-based Audi passenger car since WWII.
Even if Audi did market a rear-drive A8, we suspect the lion's share of them would still rely on Quattro, particularly here in North America.