I'm not sure it would be worth it for GM to invest the resources in designing or updating and appropriate diesel to match current and future emissions regulations. Probably an easy 2 year endeavour if not longer. They should definitely go 8 speed and try to improve aerodynamics to improve fuel economy. Other than that I don't know, maybe they can try the hybrid truck route again. If done well electric motors should (in theory) be a great fit for truck applications. In practice, who knows?