At best I could see the Nismo take a V8 or TT, but I think the standard Z would stay with the VQ. Nissan is all about consistancy, and if they keep selling the way they do then they won't stray to far from what works.
I bet the next generation Z will either use bio-fuel or be fully electric
HAHA!
Look at this electric sports car of today:
Go Electric | Tesla Motors