Stepping out of the UN is not an option for pakistan, which in the end amounts to self imposed isolation when none is required out of which nothing concrete will be achieved and I seriously doubt such a move will be allowed to happen both by the bigger powers and by the pak army which has very strong interests to be protected. One has to think on lines as to what Pakistan brings on board to the UN, the reason behind it and the replacement.
Pakistan today contributes extensively to the UN peace keeping force, now the main reason that gets done is because of the handsome salaries that get earned, and these postings are either seen as an appreciation to the good one has delivered at the home front or are delivered as a reward because the juniors were able to keep themselves in the good books of the seniors. Can this be replaced if Pakistan were to withdraw, yes, though there remains a quota to the extent one country can deploy but when faced with constraints UN can increase on the number of people that can be deployed from each country and there are many countries including India which will be more than happy to chip in.
The other and a more important aspect is a nuclear Pakistan, and that to an extent is the bargaining muscle that Pakistan has, now if a nuclear weapon armed state were to walk off the UN then that in itself would be diluting the UN to an extent which will possibly bring bad press across the globe for the UN but I feel with very strong USA interests vested in Pakistan with a polity and military completely aligned with those interests, that would be a maneuver very difficult to bring about. The down side in here if done could be that this will bring about a suspicion on the intent of Pakistan and a possibility of Pakistan being painted as a rouge state, the last thing you want to do is go against the USA’s interests in a world over shadowed by the might of the USA.
Will India get the UNSC seat? Yes, how soon? Seems like within this decade, at max with a decade and a half, had the bush administration been around the world press would have been abuzz with India being brought to the UNSC, and the sole reason of that happening will be the compulsion for the USA to do it, or else china will become a headache at the world stage for them a little too difficult to handle, and they need a counter balance.
As the things stand today, china is no where a problem one sees and they will fall in line as they fell when the nuke deal happened, and pranab mukherjee has been quoted as saying, “(Chinese) President Hu Jintao reiterated the assurance given by Chinese Premier (Wen Jiabao) to Prime Minister (Manmohan Singh) in April last year that China understands and supports India's aspiration to play a bigger role in the UN, including in the Security Council”
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The problem is the USA which wants an assurance coming from India that we will align to their way of strategizing, and be in their loop on all major issues and this is an assurance very difficult to come by though one might see a behind the door understanding but in a vibrant democracy where a regime change happens every five years that is very difficult to come about. How will this part be negotiated, very difficult to say but yes India will to an extent give in on its independent foreign policy.
Someone said, Pakistan should be brought in for being a muslim state, answer for that is, that wont be happening since that claim has been staked by the kingdom of Saudi Arabia, and there is no bigger voice for the muslims than that of the Saudis.