Given the traditional rivalry of India and Pakistan, I rather imagine that it's unlikely that they would ever have willingly allied with one-another, even if they did manage to eventually achieve some sort of peace over Kashmir and other issues between them.
I'm not sure "traditional" is the word, considering they've only existed as separate nations for 61 years and their creation was an awkward compromise imposed by Britain in its haste to get out of India. Heck, the Hindu and Muslim communities in India generally got along relatively well until the British Raj stirred up hostilities between them to keep them divided and weakened. Really, the chief rivalry is over Jammu & Kashmir, but their inability to resolve its status has just kept the fight going and created increasing amounts of bad blood. So I'd call it more a feud than a traditional rivalry.
Between that and the real-world rise of China as an economic and military superpower that everyone's expecting to occur in the next few decades, I would speculate that the Eastern Coalition is dominated by China and that either both India and Pakistan were conquered or otherwise forced to join the ECON, or that either India or Pakistan allied with China and joined the ECON willingly and that the ECON then conquered, or otherwise coerced into joining, the other.
No conquest need be involved. A coalition isn't a single state, it's an alliance of states. It's possible that India, Pakistan, China, and others decided it was in their best interests to ally despite their differences. It's entirely possible that India and Pakistan finally agreed to hold the plebiscite that's been promised and delayed for decades and let the people of Jammu & Kashmir officially decide which nation they want to belong to, or that they resolved the dispute some other way.
Indeed, China may not be the strongest member of ECON. China's Three Gorges Dam is a disaster waiting to happen, built with inferior materials and poor quality control due to rampant corruption. There's a very good chance that it will burst sometime in the next few decades and cause a flood of epic proportions, killing and displacing millions. The resultant social discontent and chaos could well bring down the Chinese government, maybe even breed civil war. The China that belongs to ECON in the 2050s may be a much weaker China with a very different government, a non-Marxist one that's therefore more amenable to an alliance with a religious state like Pakistan or Nepal.
I have no idea if Afghanistan would be part of the Muslim Bloc. I would speculate that Iran is probably not part of the Muslim Bloc because they're majority-Persian and majority-Shia, whilst most of the Middle Eastern states are majority-Arab and majority-Sunny (Iraq excepted on the majority-Sunni part). Iraq seems to be part of the Muslim Bloc from The Sundered, though I would say that that assumption needn't be hard-and-fast given their differing religious majority.
Ethnic nationalism is not a given. It's a European invention that the British and Americans have spread to the rest of the world. The Mideast today is divided between ethnic groups as a legacy of the changes Europe imposed upon it. Before then, the Ottoman Empire was a very inclusive state; as in America today, Ottomans defined themselves by national allegiance first, religion second, ethnic group third if at all. (Well, okay, a lot of Americans probably put ethnic group second.) Indeed, Islam is the most ethnically diverse religious community in the world.
The name "Muslim Bloc" suggests that its members have chosen to base their definition of identity on religion rather than ethnicity, so an Arab/Persian distinction wouldn't matter. If they placed ethnicity first, they'd probably be called the "Arab Bloc" or something. As for the Sunni/Shi'a division, that hasn't kept Muslims from banding together as part of a shared community, at least when defining themselves in opposition to other societies such as the West.
Right now, the Islamic world is going through an identity crisis, torn between different views of how to define themselves as a society. Today, ethnic and religious nationalism dominates, and the result is chaos, death, and poverty. So the next generation might reject that way of thinking and embrace a more inclusive, pan-Islamic sense of identity, perhaps invoking the Ottoman era as an idealized past to recapture. If that were the basis of the Muslim Bloc, it could easily include Iran, Afghanistan, and North Africa as well as the Mideast.
The novelization of Star Trek: First Contact by JM Dillard refers to an independent Indonesia and seems to imply that Indonesia has become a major economic power post-WW3, so I would presume that they were not part of the ECON and may have even stayed neutral during the war.
Indonesia is the world's largest Muslim-majority nation, although its brand of Islam is much more liberal and tolerant than the brand being pushed by Wahhabist militants such as al-Qaeda. It could be a member, or even the leader, of the Muslim Bloc -- although it's a society that prides itself on its religious and cultural pluralism, so it would be unlikely to choose a name like that if it were the founder. Unless it, too, went through a major unexpected change in the next 40-odd years.
