I think if you used an external battery and a shimano motor + alfine 11 hub, you could get the weight down to 23kg or so, if any manufacturers are listening.
I don't think an Alfine would be up to the job off road, the Rohloff is a proper bit of kit.
On a side note my father in law has a Bergamont city/hybrid Bike, which has a Bosch motor with an Alfine DI2 hub - it is by far and away the slickest best working Ebike drive train I have used. The Bosch electronics have the ability to interface with the DI2, so the motor know when you are changing gear, resulting in almost unnoticeable gear changes.