I hope I'm wrong, but I think it is going to be a cut-down version of the S63.
We've been hearing rumors for quite a while now that the next M3 may have a V6 with turbos. Using a cut-down S63 would be consistent with that rumor. It would also be consistent with how the S62 came about -- as a cut-down version of the V10. There also is a good business case for this approach because of the ability to use most of the M-specific components in the motor across a wider variety of models. Finally, as someone has pointed out already, a V6 would have packaging advantages, which would boost handling.
The cynic in me thinks if it is not a cut-down S63, then it will be a variant of the N54/55 motor.
We've been fortunate to see the golden age of M engineering, but I fear that age will end when the E9X M stops production.