BMW and Toyota recently inked a deal to partner on fuel cell, sports car, and lightweight Technology [more on deal here
], which should see synergies in engineering and development.
In a report
about the Toyota Supra successor, Motor Trend suggests that the agreement will form the backbone to the development of both the new Supra and the next Z4. They speculate that both sports cars will ride on a revised and beefed up version of the Toyota 86 (AKA GT86, FR-S, BRZ) platform.
Should this rumor prove true, and with the Toyota 86 receiving much critical acclaim for its handling and being a true driver's car, this will hopefully result in a much more dynamic handling successor Z4 than the current E89 Z4.
As for the next Z4's powerplant, MT's source says that BMW is testing a BMW-built V6 hybrid motor which would deliver similar power to the new Supra - speculated to be 400+ HP (from a turbocharged 2.5L inline 4 cylinder hybrid or a mid-mounted 3.5L V6 hybrid).
BMW is also said to be helping with the development of the Supra's suspension settings and handling properties.
The next Z4 is expected 2015-2016 and the new Supra comes in 2017.