The suggestion in the past has been for the DLR to extend westwards, beyond Bank, to the old Jubilee Line platforms at Charing Cross. The timescale for this is unclear, though TfL and the DLR both told me it's well into the future, namely 20-25 years. The tunnels in the Charing Cross area would need to be enlarged to take the DLR trains and a side walkway, as well as the refurbishment of the station tunnels and provision of lifts.
This is probably easier than trying to go to Euston, simply because the station tunnel(s) already exist, though they need enlarging. This option is also better from a passenger point of view, as currently from Euston or Kings Cross, to reach the docklands requires only one change of train (Northern Line - DLR at bank) wheras going from Charing Cross to the Docklands requires two changes of train, either Northern or Bakerloo from Charing cross to Embankment, then Circle or District to Monument, then walk to the DLR at Bank.
So, making the DLR extension from Bank to Charing Cross would be better for the passenger.