IF BMW goes for a V6, then that is a major change in engine architecture for a BMW 6 cyl. Would be really interesting if they did that, and would also mean that it would be a "M only" engine
But, somehow I am inclined to lean towards a I6 engine at the moment (based on some sources like VIN and photos of the engine bay - If not an I6 then BMW has really done a lot to make us believe it, with a test mule that clearly has a I6 engine in it with the bonnet open... That would actually be awesome if BMW built a I6 car for testing and media leaks and then reveal a V6 M3/4
)