A smooth V6 is definitely possible with a 60-degree V-angle. So they can make a smooth V6. However, BMW clearly prefers to set the turbos inside of the V in V-engines (as seen on the 4.4 liter bi-turbo) and I have troubles imagining that they could acheive this arrangement with a 60-degree V.
Lets say they went to 90-degree V angle and did put the turbos in the V, then that is fine, but they would do it with a sacrifice of some smoothness and free-revving nature that BMW is known for. I would be surprised if they do this. My personal opinion on a V6 tri-turbo is that, although technically possible in terms of plumbing with the turbos in the V (not possible with turbos outside the V) I am pretty certain that the heat would be too much of an issue to pull this off.
This makes me think that an inline-6 is still most likely. BMW will probably want to make this the first full M-version of a turbo so I can see it being significantly different from the N55... maybe a tri-turbo is in the cards. I think that the most likely thing will be M making their own 3.0 or 3.2 I-6 engine and then pumping the hell out of it with twinturbos. To be clear, I don't think that they will use an N54 or N55 and simply modified it a little this time like they have on the some of the current M-cars.