Please do correct me if I am wrong, but to my understanding TfL wants to build both bridges.
Furthermore, today's news of TfL taking over suburban rail services, suggest London Overground should be reaching the area in the not too far future.
The Belvedere bridge appears to be close to Belvedere station south of the river.
So what do people think of an Overground service running from Cannon Street/London Bridge, Deptford, Greenwich, Maze Hill, etc. to Belvedere, where the trains will then go over the Bridge to Rainham, Dagenham Dock and terminating at Barking Riverside.
A second Overground service from Cannon Street/London Bridge, New Cross, St Johns, Lewisham, Blackheath, etc. could loop around the Erith Loop to Belvedere and cross the bridge to Rainham, Dagenham Dock and terminating at Barking Riverside.
In this scenario, work would need to be carried out at Belvedere to allow both overground lines to connect and cross the proposed bridge, as well as create an interchange between the two lines I proposed. Barking Riverside would also need to be constructed in a way that would allow multiple lines to terminate there. A relatively small amount of track will need to be added over and on either side of the bridge to connect the lines I suggested.
I think this idea would connect East and South East London to Central London and also links to Crossrail.
With Gallions Reach tunnel/bridge, I feel a tram from Beckenham Junction to cross the bridge and connect to Barking would be the "greenest" option and connect a larger area of South London to the South East (of London). I would personally like to see more tram in London, but I don't think I will have much support for that. So I am not opposed to the OP's first post with the DLR map.
(My first post! Be nice! =])