Rotation acceleration would be very handy for quickly adjusting to changes in rotation, where the gravity vector might not be sufficient.
There’s also some algorithms that can integrate 6 or 9 axis information to give really good orientation and improved dead-reckoning positional information.
I’m prototyping a version of the Pico with a 6 axis (accel +gyro) for use in poi / staves / hoops.
A future version of the sensor board could use this too.