CALLING WILDPAWS!!!
I don't think I'd consider a 1st Gen SS a base car with Z28 parts because probably the most notable highlights of that generation Z28 was the little DZ302. You couldn't get that engine in an SS, or any other model because the torque wasn't there to move the car, so-to-speak. If I remember correctly, one couldn't even order a slushbox with the 302 because of the lack of low-end torque. That's not to say you couldn't swap converters with a TH350 or 400 and get all crazy at the strip, but I believe there were only M21s and M22 4-speeds available for that engine. The smallest engine in the SS was a 327 or 350. The other big highlight with the Z28 was the chassis; available disc brakes and other goodies made it handle at the track really well. The SS was the 1320' and street bruiser. You couldn't get anything but the 302 in Z28, while you could go as high as 454 (at the very end of the 1st Gen if I remember correctly - LS6) if you knew the right people; I know, for sure, there were L72 427s, L78 3696s, and ZL1 427s to be had too.
More to you question, probably no one would know, outside of GM. I think the SSs from the 4th Gen were REALLY popular; I don't have production figures handy, but I'd be willing to bet there was a higher demand for them than Z28s, mostly because of body and some subtle chassis refinements. This time around though, I just don't know why. I'll be the first to admit I recognize the Z28 it looks like we could likely see will not be a blast from the past, in that it's going to be heavier and have more power than the SS. Shoot, if it handles better, that might be a miracle, especially if they can keep the price reasonable to most regular Camaro-nuts. I think the SS fell in line with the rest of the Chevy line, being they had Cobalts, Impalas, Silverados, Trailblazers, all with SS credentials. Maybe Chevy thought they could grab more attention, right out of the gate. Personally, I think that was the case, and that they wanted to release a higher performing car a few years later. It seems, more often than not, that the Big Dawg model usually comes out a little later than the more mainstream models (except for the Challengers). Anyways, I think it's fair to say that Z28 will, at least, be a higher-performance car, considering the Mustang GT with the Track Pack does so well; and I won't even mention the elephant in the room... Whos though? It's definately safe to say that even if it doesn't come back, that we have one Hell of a Camaro with SS and I'd be pretty happy to roll one of those babies too
JMVHO.