With the upcoming launch of the baddest Camaro ever, GM isn't going to add in anything to distract from a car so close to being able to be ordered and likely, at least initially, will be high in demand and interest.
Recall, GM is a business that needs to sell cars to survive as the bottom line. We WANT GM to succeed, so you don't want to . And marketing gurus know that if you really want people to want something, you hold it just out of reach and make available slightly less than the demand, at least initially. Otherwise, you'd might as well have a Firebird too competing for your dollars. GM isn't going to do that, so we have to wait.
Once the ZL1 is in the showrooms for a bit, I'm sure there will be discussions of a new model/edition on the way, and Z/28 seems like a natural fit, but we'll just have to wait and see. I have faith.