The latest future planning for the DLR is to extend the service to Dagenham, Euston, Victoria and Forest Hill. A good analysis of these early options is presented at http://londonreconnections.blogspot.com/...g-dlr.html
The idea of extending DLR from Catford to Forest Hill seems a strange idea as it would either require a tunnel or a viaduct built above the South Circular, and there would be little additional benefit to people in the centre of Forest Hill as the ELL provides faster connections to anywhere north of the river.
My preference would be to terminate the southern extension at Bell Green. Providing additional transport options to one of the poorest parts of Lewisham - Bellingham ward - on both sides of Bell Green. Lower Sydenham station could be moved to Bell Green with fast services to Charing Cross as well as the DLR to Canary Wharf.
Forest Hill (and the area of West Lewisham) still requires significant additional capacity to meet demand in the next couple of decades, but this is best provided by the Bakerloo line extension which can terminate at Bell Green as well, having come from Forest Hill, Honor Oak or Crofton Park. And before that we need longer trains on ELL and Southern, and more frequent services where possible. Full details of my submission to Network Rail on behalf of the Forest Hill Society can be read here.