The Z5 rides on a platform which is being co-developed by BMW and Toyota. Toyota is also expected to build its own sports car based on the joint platform.
The Z5 is supposed to spearhead a revival of the Z model lineup which will feature more dynamically impressive and exciting roadsters. The return to a soft-top as seen on this prototype represents a focus on lighter weight, which in addition to the car's mix of lightweight composite construction, should see the Z5 come in at a lighter weight and provide better performance than the Z4.
The future Z lineup is rumored to also including a Z3 and a Z1 roadster. The Z3 will share the Z5's modular RWD platform and the Z1 will be based on BMW's UKL FWD/AWD platform.