If you are considering a re-design of the bottom brackets, may I suggest cherry-picking this concept?
http://www.thingiverse.com/thing:340529
![frame_motor_2020_QD_ASS_preview_featured.jpg]( "frame_motor_2020_QD_ASS_preview_featured.jpg")
Instead of slotting the motor holes, just make the entire green plate a little bit shorter and slot the extrusion mounting holes so the whole plate/motor can slide to tension the belts without disturbing the frame geometry. My only concern is the ease of access to the front (shared) extrusion mounting holes with the motor installed.....
http://www.thingiverse.com/thing:340529
Instead of slotting the motor holes, just make the entire green plate a little bit shorter and slot the extrusion mounting holes so the whole plate/motor can slide to tension the belts without disturbing the frame geometry. My only concern is the ease of access to the front (shared) extrusion mounting holes with the motor installed.....