Wouldn't the GNX just end up being a meh V6 FWD/AWD sedan?
Depends on who you ask and where the rumor mill has been turning.
The last GN/GNX rumors were banking on GM pulling the trigger with the Avenir and Avista, both of which would be easily adapted to a high performance option given their relationship to the Cadillac CT6 and Chevrolet Camaro respectively. Buick has stated that neither are in-line for production, which of course, has a bunch of GM fans rather upset. I can certainly understand why the Avista wouldn't happen, after all, it would very likely cannibalize sales of the ATS (which has already had a tough enough time on it's own). The Avenir on the other hand has had its styling adapted in the new Buick LaCrosse, and while I absolutely believe there is room for a full-size luxury, rear-drive sedan at Buick, again, it may step a little too much on the toes of the forthcoming CT6.
So, that leaves us with the more likely option of just continuing on with the Buick GS for now, very likely transferring to the next-generation Regal. The E2XX chassis has been pretty well accepted as a halfway decent chassis, it will be interesting to see what Buick (ahem, Opel) comes up with for an Insignia replacement, and whatever that OPC/VXR/GS variant ends up being.