Either way, the speculation is interesting, and considering the lack of info coming from Corporate, it's really all we've got until some sort of "official" announcement is made...
Of course, we'd love to think that Nissan would choose to let the web's largest Nissan site in on the "secret" before anyone else, but based on past occurrences, we're not optimistic. It'll probably pop up on some obscure non-enthusiast site or in a Facebook teaser.
Anyhow, let's kick this rendering around... Could this be an accurate depiction of the Z35?

While you're considering the image, let's talk about what we DO know so far.
[NOTE: Obviously, this is all conjecture at this point. However, we get bits and pieces from our contacts within the company, and our staff spends a lot of time discussing what COULD be done with the company's current platforms, powerplants, and resources. Filter this all through the Nissan three-prong fuel economy strategy (lighter weight, lower drag, more efficiency), and we get a pretty good idea of what we'll see in 2016.]
What do we know?
We already know the next-generation Z will have to be downsized from the current 370Z, which will mean it'll be sized similarly to the original 240Z. Based on our earlier discussion of Nissan's new lightweight high-tensile steel, plus Nissan's commitment to making ALL of its lineup lighter in weight, we can anticipate the car being in the 2800-lb range.
Nissan has been pushing hard to retire the current crop of V6 powerplants across its lineup, which means we'll likely see an efficient, high-horsepower 4-cylinder. If Nissan follows the lead of its competition (think Hyundai Genesis Coupe), we could see a pair of engine choices - A naturally-aspirated 3.7-liter V6 and a turbocharged 2.5-liter 4-banger, with direct injection and lots of efficiency measures built in... likely in the 280-hp range.
Of course, that leaves the door open for a Nismo Z variant, which would (we hope) pack bigger horsepower and hopefully some track-inspired braking, handling, and appearance goodies.
Ready, set, DISCUSS!

