GM currently has two RWD large car platforms, the premium Sigma architecture used by Cadillac for cars like the CTS, SRX and STS, and the newer Zeta design, which debuted in the new VE Holden Commodore (Pontiac G8) and will eventually appear in the upcoming Chevrolet Camaro. According to latest reports GM may merge the two designs for Cadillac’s single vehicle replacement for the STS and DTS models.

The new Cadillac sedan is due in 2010 for the 2011 model year and its design is close to being finalized with only the car’s front-end still being worked over, reports Motor Trend. The new merged Sigma-Zeta platform will come in three flavors and all with forged aluminum suspension for a premium road-feel and better handling. The trio will include a shorter ‘Coupe Premium’ design earmarked for the next-gen CTS coupe, a ‘Midsize Premium’ for the next-gen CTS and a ‘Large Premium’ for the new STS-DTS replacement.

GM is also working on a third RWD platform for a range of new small sedans including a new Cadillac rival for BMW’s 3-series that’s aimed at European markets. A non-premium version of this platform is expected to spawn vehicles for Holden, Pontiac and Chevrolet.

The next-gen Chevrolet Impala won’t be getting a RWD platform. It will likely feature a stretched version of the FWD Epsilon II platform that debuted in the new Opel Insignia and will also appear in the next-gen Buick LaCrosse.

2009 Opel Insignia

2010 Buick LaCrosse spy shots

Buick Invicta Concept