Almost certain the S65 turbo powerplant will not happen.
The more certain options for BMW are taking their N54 and N55 variants and doing something real with it this time unlike the 1M.
If they shoe-horn the S63 motor into the next M3, then that is enough proof that the M3 is most likely going to gain weight and only get bigger. This explains many of the things the BMW M engineer I spoke to in Germany was saying.
Before owning the current M3, I always felt the last true M3 would be the E9x platform. No matter what the reviews claim the new 1M to be, it still has the N54 platform which pails in comparison to the true sheer driving pleasure the S65 provides. No matter what was done to the N54, it will still boring, mundane and too civilized no matter how fast the car. I know this may tick off a ton of the 1M owners, sorry in advance.
The current M3 is truly a blissful experience that has me giggling from time to time, even when I feel like I have pushed her the wrong way and about to lose it. The engine response is immediate and makes throttle correction amazing. Fact is, from a power delivery standpoint, no turbo engine with a single butterfly will ever have the response of a high strung ITB NA engine.