Get on with gearbox/internal type systems and make them light, weatherproof and reliable.

Out of curiosity I started going through the design process once for a simple two speed hub. Once you start looking at things like the tooth forces involved you soon realise light and reliable is never going to be an option.
Bastardising Winston Churchill –
“It has been said that deraileurs is the worst form of gear shifting, except for all the others that have been tried”

Get on with gearbox/internal type systems and make them light, weatherproof and reliable.

You can have two out of three of those – and a Rohloff or Pinion isn’t all that heavy.