I'd be surprised if they went significantly smaller with the next Z, since they're developing a new 240SX-type car to compete with the BRZ/FRS/GT86/KDIG)/@#KG/LKA in that segment. (Finally starting to see some of these on the road where I live. They're certainly better looking in person...in my opinion.)
I think they'll stick with a V6 to stay in line with the Genesis and V6 muscle cars, even though some of those (Ford) may be going Turbo 4 cylinder.
Everyone wants to shed a couple pounds. I'd just like to see more refinement, which I'm sure they'll do in terms of driving comfort/daily usability. Oh and more torque, dammit.