Thr late FJ Camm, editor of Practical Mechanics/Wireless/ Television, brother of Sydney Camm who designed the Hurricane WW2 fighter was a keen cyclist and suggested that the best place for gears was the bottom bracket. How about a Rohloff system in the B/b?
Advantages would be many- weight centralised, short cable run inside tubing, simple chaincase or shaft drive, possible coaster brake incorporated, easy lube down seat tube, and I am sure many others.