Good luck to Mazda but I highly doubt it will succeed. There have been other, mid-size "RWD platforms as a BMW/Mercedes alternative", with greater resources than Mazda (Infiniti Q70 and Lexus GS come to mind, even Jaguar and Maserati) and they have all failed to be successful against BMW and Mercedes-Benz.For me the good news is RWD platform as a BMW/Mercedes alternative and more than 4 cylinders.
I personally believe that a mid-size RWD Mazda sedan is the wrong product for the North American market. BMW and Mercedes-Benz own the mid-size RWD sedan market. Toyota and Honda own the FWD mid-size sedan market. All other automakers may sell handfuls of mid-size sedans but those resources may be better spent on a mid-size (FWD) crossover (I believe that buyers of mass-market crossovers really do not care whether it is FWD or RWD but a FWD has packaging and daily driving advantages over a RWD package). It may be a better bet for Mazda to give up the mid-size sedan market (like Ford and Chevrolet have done) and spend its resources on a good, mid-size 2-row crossover.