Pakistan at that point would also include Bangladesh, since I'm assuming you're meaning really early.
Problem is- In OTL Pakistan tried several times to get CENTO to draw in other countries in the Indo-Pakistani wars, but in an ATL where Pakistan is realistically going to lose and be annexed... This might actually persuade CENTO to activate and you'll have Iran, Turkey, and the UK get involved. At the very least certainly Iran will, even if the consequences are that Iran annexes or puppetizes Baluchistan keeping that part out of India. Afghanistan might make a grab for the Northwest Territories.
India will have to deal with a large Muslim population that because of the population transfers from 1948 is more Muslim/less Hindi than before partition. Do Hindis from before partition get the right to move back "home" and take their land back? Do Muslims get the same right? This entire population exchange and now a united India causes huge problems for Bengal. How it evolves depends on what you say India does as far as civil rights for Muslims and how the former Pakistan is treated, as an integral non-biased part of India with full rights? Or do you have those territories have less rights and more restrictions?
Its early enough that China's not a threat, although does a united India affect China later invading Tibet? Can India stop it this time? India's relations with the US and USSR? The US is not going to be happy losing Pakistan as an ally to a nation that has been too nice to the Soviets compartively speaking.