Just my .02, but the engine will consist of a V6 with a twin turbo set-up. I'd guess the hp won't be drastically more, at max 460/470, but a significant bump in torque, around 360-370. Also a weight drop, from 3725 to about 3575-3625lb.
I personally think we'll see some more use of carbon fiber given BMW's massive investment into carbon technologies, but also resulting in a nice $3-5k bump in MSRP.
Just my guess, so let's see if I'm an idiot or a genius