Now that you've clarified your position a bit better, I get what you are saying, Croc. In fact, I largely agree with you that Buick should primarily remain FWD in counterpoint to Cadillac's primarily RWD lineup.
However, There is ample room for differentiation between this Alpha Buick halo car and the ATS. In fact, I see it fitting perfectly into the GM portfolio while providing much-needed brand identity for Buick.
You ask what I would do with suspension bits and interiors between the two. Well, the problem here is that what the ATS should be is a bit beyond the scope of the discussion of a new Riviera.
I don't want to get too deeply into what the ATS will be, but I think that we all know that the interior and exterior design will be all A&S as we see it in the CTS. Keeping this hypothetical Buick different on those fronts is simple if the design is based on the render I posted - all of the shapes inside and out would be very different.
As for suspension, think of the difference between Benz and BMW. Those two feel worlds apart in driving dynamics yet the structures are just variations on a theme. So the Buick would be tuned for a Benz-like ride, and the Caddy like the BMW.
Even the drivetrain choices could be different in that Alpha is likely to be package protected for a V8 (for Camaro), so a V8 ATS-V might even be possible.
So, if GM builds the ATS,this Riviera,and the F-6 Camaro on Alpha - I think it could be the best example of platform-sharing with differentiation in GM's history.
With those three cars, they could really show how it is supposed to be done.