All information comes via TfL, because they haven't yet published the geological/engineering studies they undertook that suggested the extra build cost and disruption for the Tooting Broadway station.
I do know that for now only options that join up directly with the Northern line are being considered, so as to relieve Tube overcrowding problems. Tooting Bec was suggested, but rejected because it also has an underground seam running beneath it just as Broadway does (but Balham doesn't).
I have a number of requests out for further information on this and will report back when I hear more.
In the meantime, please do encourage other local residents to give their initial views to TfL via
http://www.balhamortooting.org.uk/.