As a lover of singlespeeds and hub gears (and with a belt fetish too) the biggest problem I’ve had setting bikes up is chain (belt) line, mainly with external bottom brackets and cranksets as the chainline can’t be altered, except +/- 2.5mm by switching the spacers around which moves the whole crankset.

At least old cartridge BB units had different axle lengths (I’m using a 110mm axle on my Alfine’d Singular). So it go me thinking, would a 3-piece crank allow different axle lengths, can you get them which work with a “standard” external BB and would it beneficial to allow different Q-factors to be used?
And does anyone make “lightweight” 3-piece cranks, as opposed to the monstrous BMX ones?

That make sense?

I think Profile now do an external bearing BB, as may Gusset? Not sure about the ability to mess about with chainlines etc with them though although I don’t see why it’d be any different to normal BMX style BBs which you can by messing around with the spacers either side of the BB cups. As for lightweight, you’d be struggling I think to get anything close to XT etc weight. Primos new version of the old Hollowbite might be worth a look… not really sure if theres much lighter. You could look at Ti axle etc.