Although I think it has already been described as impracticable on here, I hoped the DLR could be extended from Bank to Waterloo, taking over the W&C Line.
That would be a bit pointless - it would just be a (longer) duplication of the Jubilee Line!
Plus, I think it would be one tight/sharply graded curve to join the two tunnels together (without taking out the Northern Line, which lies directly above the DLR at Bank).