I have a manual 2SS and absolutely love it. Aside from adding more power which the Z/28 will address, my only two suggestions would be:
1. Navigation: add a real touch-screen navigation. Although OnStar offers it through their service, this is not a long term solution for most owners. It would not help us if we decide to discontinue Onstar nor is it cost effective for an owner to keep their service just for the navigation feature.
Just from a competition and market share standpoint, the mustang offers this in both the GT and GT500 so that could be a deal breaker for a present Mustang owner or someone debating between the two models.
2. Grill: Start making the Heritage Grill the standard on the SS or performance models. Not only is the stock grill cheap looking, but there isn't enough differentiation between the SS and non-SS models.
Just my $.02.