Given there is significant political (rather than passenger benifit IMO) motivation behind the extension, similar to that of any more bits of DLR into deprived areas, I don't think anywhere but Clapham Junction will have been considered. If it where to extend to Clapham Junction, with the inclusion of Crossrail 2, it would potentially also need to include an extension of the Victoria Line with a new branch in order to cope with demand. Even then, I can foresee every train being full by Kennington and Victoria, with Waterloo and Victoria having some manner of relief as a result, but this would need rather significant modeling (with some sense applied) as to if it would work. In addition to this, I think consideration needs to be given to if the surface platforms at Clapham Junction could cope with all of the additional interchanging passengers that are waiting in the peaks, at the moment, most peak interchange passengers will be heading towards London where you'd board the first train in that direction, when you could be waiting up to 30mins for your train at Clapham, rather than waiting on the concourse at Victoria or Waterloo, could Clapham Junction cope with it? I honestly do not think so.