None of the future engines that Nissan has patented is a 3.9. It will either be a 2.5 supercharged (which I doubt) a 3.0 and/or 3.0TT(most likely. Or a hybrid 3.7? Which I can equally see happening.
The 2.5sc I think will bring forth the Datsun Silvia.
just my guess.
I can actually see nissan dropping the numbers and calling iit the Nissan Z and bringing forth all engines as options in different price ranges... you never know?