Good to make provision for a river crossing. But it's only about 2km downstream from the newish DLR and forthcoming Crossrail crossings to Woolwich, both of which offer a better range of destinations, I can't see building it as a high priority. I wonder if there is scope to extend the DLR from Woolwich through a selection of stations in Thamesmead to finish at Abbey Wood.
As others have pointed out, the DLR at Woolwich is facing the wrong way, so it wouldn't really be an extension - more like a new DLR line. It's an interesting idea and would have the advantage that you could put several stations in Thamesmead, thus serving the complete area very well. However, I could see it being very hard to link it to the existing DLR at Woolwich. Woolwich Arsenal DLR is underground, and, after leaving the station, the lines head straight into the tunnel under the Thames. I imagine any attempt to build a junction there so lines can run off to Thamesmead would be extremely expensive, and would involve closing Woolwich Arsenal DLR for an extended period.
The other problem is that you'd end up with a new DLR line that just runs from Woolwich to Thamesmead. While you rightly say there are a lot of potential destinations at the Woolwich end (although not
that many more than would be provided at Abbey Wood for an Overground extension), you have the problem that there is only one possible destination at the Thamesmead end: Thamesmead. So the only patronage would basically be people who live in Thamesmead - no scope for through journeys to other places. That contrasts sharply with the proposed Overground extension, which has a lot of possible interchanges at both ends, and I imagine it could be a problem for any business case.
Woolwich station is facing the wrong way. You would be better serving Thamesmead by extending the DLR from Gallions Reach by building in a DLR route on the proposed road bridge at Gallions Reach and re-routing the Stratford International to Beckton service (when reinstated with a bigger fleet) to Thamesmead.
That would probably work well - it would give good interchanges at both ends of the route. And it certainly strikes me as a better use of money than building a road bridge at Galleons' Reach. But I'm not clear what advantages it would have over an Overground extension, especially given that CrossRail will already provide a fast Abbey Wood-Excel link.