As we are all aware, India, Japan and other countries those who are potentially eligible for permanent members in UNSC being denied to become so in spite of their active participation.
Though India had turned it down during 1955 by the reckless foreign policies of Nehru and his advisors, can UN & US do something now in 2015?
I am not sure Nehru's politics had that much to do with it. Current permanent members have very few reasons to add a new one and even talking seriously about India joining would open a huge can of worms.
What about Japan as you mentioned? What about Germany (not nuclear – but Japan isn't either – but a large European state that certainly wants to see itself on a par with the UK and France) or Brazil (large state too and vocal about the need to broaden representation)?
If you add those four, it will be even more difficult to get anything approved at all. And then you have to contend with the opposition of the “next in line”, countries like Spain, Pakistan, Mexico, etc. that might not be quite as large or powerful but have reasons to be skeptical towards their neighbours and their ambitions.
Importantly, the UN Security Council was never intended as a sort of fair representation of anything nor is it a reward for some sort of political significance. It exists for one reason only: Without it, the current permanent members (which include what were at the time the two superpowers and two large colonial empires) would never have gotten on board.
By contrast, India is in the UN (quite happy to participate in its peacekeeping missions, incidentally) and not going to leave anytime soon. So it has very little leverage.