It'll be either the an N55 variant or the v6 probably. BMW has already committed to their efficient dynamics ploy. And since that's the case it's going to require cars to weigh less.
I'm actually betting on the v6 bi or tri turbo. Fan boys will cry then they'll drive it and thus the story continues... Just like the v8 haters 5 years ago.
1997 M3 Coupe - BMWCCA I-Prepared #22
2006 M Roadster - 23,000 miles and slowly counting