Kris Winer of Pesky Products is creating modular add-on boards for the Teensy 3.x:

Modular Teensy Project

The intent is to gain 10-DoF motion sensing capability using open-source Madgwick/Mahony sensor fusion in a very small package

One of these 9-axis motion sensors:

• Invensense’s MPU9250
• ST Microelectronics’ LSM9DS0
• Bosch’s BMX-055

is coupled with one of these altimeters/pressure sensors:

• Freescale’s MPL3115A2
• Measurement Specialties’ MS5637

Kris writes that the modular approach allows other capabilities such as Bluetooth Smart, LiPo battery charging, and motor control boards to be added to the same Teensy to provide flexible configurations tailored to specific applications