I rated Rosberg highly before he moved to Mercedes but even then he managed to up his game an awful lot when Hamilton joined, and I think it surprised people that he challenged and beat Hamilton as regularly as he did.
I have doubts a returning Schumacher could have matched Rosberg, but if anyone could have upped their game if a title was up for grabs then I can think of few people who would be better suited than Schumacher.
He was never going to be as good as the Schumacher of old, but it would have been interesting to see just how resilient he could get with the new era - in terms of driving style it was after all a reset for all of the drivers, not just Schumacher.