This has been discussed at some length on a thread over at Skyscrapercity, in the context of extending the ELL from Clapham Jn. The idea is not bonkers, but perhaps should be viewed in the context of Crossrail 2.
If Crossrail 2 is combined with new infrastructure to relieve the SWML and the Windsor Lines, then capacity could be released on the Windsor Lines through Wandsworth Town. Or, this could become a branch of Crossrail 2, so would help relieve the District.
However problems still exist on the 4-track section through Wandsworth Town and the bridge over the Wandle to the west, and with the flat junctions at Clapham Jn.
Another thing to throw into the mix is a putative extension of the Northern extension to Battersea, to Clapham Jn and perhaps beyond. But that appears to be equally pie-in-the-sky.
I suppose the real issue is how such potential routes are safeguarded, so that they may still be possible in the future...
Just to complicate things a little further, don't forget the unusual arrangement currently on the Windsor Lines Clapham Junction to Barnes arranged as: Up slow Up fast Down slow Down fast So swapping the Down lines back to their old configuration to part-enable this idea would cause a re-timetableing of the current situation anyway...
Services from the West London Line would have to do a heck of a lot of points crossing just to get to the line to Wimbledon via Earlsfield (assuming they'd call at Earlsfield). Infact, I don't think the current station layout at Clapham Junction even allows for trains to get from Ludgate Junction to Platforms 10 and 11 at Clapham Junction, ......... Even if they used the flyunder which Southern Services use to get to/from platforms 16 and 17, that's too far over to access the South West Main Line towards Wimbledon.
As I understand it, the only ways to get to the SWML from the West London or South London lines at Clapham Junction without reversal are via East Putney, via Addlestone, or via Balham and Haydons Road
When reading this, is a platform 0 at Clapham Junction likely to be created?
I suspect that the eventual re-instatement of the Northernmost platform at Clapham Jct is an inevitability,the only consideration being money. The reason it wasn't done in the first place was the condition of the Banana Arches below,and the metalwork which supports the track. What we have now is a provisional solution,and a change in parameters (more frequent/longer trains,say) will expose its shortcomings and compel those in charge of such things,to bite the bullet.