If we look at this logically which means looking at package size, weight, tune ability then I would say the logical step for BMW is in fact a twin turbo V6. Especially considering that they have mastered having most of the plumbing/hardware inside the V like in the M5 V8. the V configuration takes up less space and allows the motor to be moved further back. Plus if you listen to the videos there is a distinct GTR sound to it, which differs to the sound of an inline motor.
If I was a betting man I would put my money on the V6.