Just throwing out the question, other than tradition what are the compelling reason(s) for BMW to keep the boxer engine? The advantages BMW always touted were its cylinders were both out in the airstream for efficient cooling and the inherent primary balance of the opposed cylinders. It's now going fully liquid cooled and with modern counterbalancing it's not necessary to have opposed cylinders for smooth running. And with the crankshaft up so high it doesn't provide a particularly low COG. It's more expensive to manufacture than in-line motors. It does offer a longitudinal crank for shaft drive but so can a parallel twin or in-line triple.