Interesting topic. I think we will not see an V12 anymore and most probably a smaller displacement.
I also would tend to agree that a hybrid power train would be more appropiate but i have no idea with what engine they could hook it up, it should be a new designed engine for sure.
If you look at McLaren, all their cars have basically the same engine, the 3.8L TT V8. That being said i would prefer a high revving V10 over an TT V8.