If, and that's a big IF, BMW has a need to put a six cylinder in any of the upcoming FWD cars (including Mini), I can see them justifying the need for a V6. But I just don't see why they would want a V6 in a FWD car, since the whole point of that class is to have a smaller, more space efficient and fuel efficient model, so I'd think they will be limited to 3 and 4 cylinder engines.
And as far as pedestrian crash regulations, they've already managed to get the I6 to work with the F10 5er, and the F32 3er, without making them look horrendous (at least the F10 5er, since I haven't seen a clear pic of the F32). So, I'd bet that there's no V6 in the near future.
What I do wonder is how Porsche has managed to wring out 400+ HP from a naturally aspirated flat 6, along with good torque figures, and have done so reliably, yet BMW has chosen to drop that from the equation.
I truly feel that if it's strictly the MPGs they are worried about, then a lighter chassis (by about 300 lbs), is key, and they can go to a 3.5 L I-6 naturally aspirated that puts out 420 hp or so, and have a car that's as quick as the E9X M3, but more fuel efficient. However, I think they would lose the war on paper with MB and Audi, as their numbers would look better, despite the fact that a 3300 lbs M3 with a 420 hp I-6 would be one smooth and agile handling car.