It looks like BMW is moving forward, albeit at a leisurely pace, with plans for a successor to its discontinued Z8. According to InsideLine, engineering studies are underway for a more "mass-market" roadster, likely to be tagged the Z6, and targeted at the Mercedes-Benz SL.
Apparently, the Z6 concept has its roots in the California Spyder design study (shown above) unveiled at the Design Los Angeles exhibit at the 2005 Los Angeles Auto Show. Developed by BMW Group DesignworksUSA, the California Spyder was conceived as combining BMW driving dynamics with luxury and presence suitable for the L.A. scene. The concept included a folding carbon-fiber roof, and apparently a folding hardtop is still considered an attractive possibility, although we're guessing that a metal version is more likely than carbon-fiber.
The Spyder was envisioned as being powered by the 3-liter V10 used in BMW's last-generation Formula 1 car, but a production version is likely to draw upon BMW's current road-car engine portfolio, with the possibility of an M-series Z6 driven by the M5's 5-liter V10.