I for one think BMW won't let people down.
The next gen M5/M6 will use a modified version of the X5M and X6M engine. I don't see anything wrong with that.
The new 5 series engine lineup already paved the way for the next 3 series. A straight 6 single turbo will be the top end 3 series engine. The M3 will get a turbo V-6 based on the X5M/X6M engine, probably twin turbo and that will be the unique quality the M3 will have...being V-6 vs Inline.
It's not that hard to predict but hell, we can all be wrong and BMW will use electric motors and a bio diesel engine
One thing is sure, it will drive great.