Hopefully Nissan understands that the next Z can't be a refreshed version of the current model. If that is all they plan, they might has well take another decade vacation. I'd like to see them find a few hundred pounds of weight reduction and go with a 2 engine solution. Keep something like the current engine (or turbo 4) for the lower priced end and then something like this engine for the high end.

Mercedes CLA 45 AMG is gonna have a turbocharged 2.0-litre four-cylinder engine with 355bhp and 332lb ft. Maybe the Z should get that engine. Bring the weight down with a 4 cylinder. how much weight could be saved from moving to a 4 cylinder from a 6 cylinder? Educated guess.

Nissan released this already. It's called the 300ZX Twin Turbo. Well with a new ECU tune and stage 3 upgrade anyways. ;)

Seems to be a fair amount of hints out there on this.
Mercedes-Benz and Nissan Join Forces to Build 4-Cylinder Engines in the USA - Carscoops

....A third project involves a comprehensive powertrain cross-supply that will see the Alliance giving the Germans 3-cylinder gasoline engines for the Smart and 4-cylinder diesels for their jointly developed LCV and Mercedes-Benz's next generation of premium compact cars, while Daimler will provide Nissan and Infiniti with four- and six-cylinder gasoline and diesel....