Unrealistic, even as alternate history.
The closest I can come up with is the GCC uniting under a single flag, with Riyadh as the capital of this new nation..The King of KSA becomes a constitutional monarchy, as well as other Arab monarchies. The new federation (let's call it "the Gulf Federation -TGF-"), becomes an economic powerhouse, becomes the new top regional power. The newly democratic Islamic federation, with it's new found wealth, starts investing in industry to provide jobs and increase industry, as it is more domestic politically driven than it's monarchic predecessors.
Population
• 2014 estimate 50,761,260
• Density 17.37/km2
45/sq mi
GDP (nominal) 2013 estimate
• Total $1,665 billion
• Per capita $33,005
On the new TGF military and separatist rebellion:
Assuming it survives the various secessionist rebellions (mainly from Qatar) that will inevitably occur, It will use it's wealth and influence to further isolate Iran. It's new found nationhood allows the armed forces of the individual states to finally fully integrate under a single flag and single doctrine, this also leads to better training, coordination, and standardization of equipment between the different states within the new federation.
On taxation, oil and new TGF nationalism:
Fears of running out of oil, allows for a political consensus to push for investment in more renewable energy sources. Where as taxes were relatively low to nonexistent, the new government, wanting to get decrease any deficit, and create a strong financial buffer, enacts new taxes to increase the national budget, and national coffers. The new taxes (while unpopular) are accepted by the population, as TGF nationalism is high, due to federal forces winning out against the secessionist forces, and the high gdp growth that TGF is experiencing, again due to it's wealth.
On Pakistan's relationship with TGF:
Pakistan is no longer needed for military aid, though training and advising continues, as such political and military contacts are maintained. The federation becomes Pakistan's third largest trading partner, after the US and China. With Gwadar (being backed by China), the TGF proposes connecting Gwadar with it's own ports, so trade can flow easier; Whether this goes through is unknown.
(note: this is kind of fun, alt history always makes my mind rush with new possibilities.)
Assuming the crisis Yemen and Syria occur,
On Yemen:
In our timeline, Yemen was in negotiations to join the GCC; In this new timeline, Hadi agrees to join as a new state within the federation, in exchange for governorship, and help against the Houthis. With better coordination, training, and equipment standardization, the Houthis are forced to at the very least, agree to talks, whatever the outcome, Yemen joins. Because of Yemen's underdevelopment, and large population, TGF suffers an political and financial crisis, as such integration of Yemen into TGF is slow, while TGF has no choice but to invest heavily into the new TGF state of Yemen.
On Syria:
TGF armed forces declare a no fly zone on top of Syria, giving a warning to Russia not to interfere in Arab affairs, promising to invest in Russia if it withdraws, and threatening to place sanctions if it doesn't. With Russia's already weakened economy, Russian support for Assad declines from military to political. With a no fly zone declared, and Russia out of the picture, Assad's forces suffer major loses. Hezbullah is forced to withdraw, and TGF forces successfully blockade Syria, which causes Iranian supplies flowing to Hezbulllah, to decline.
With the rebels:
The new federation is actually far more moderate than it's predecessor monarchies, that makes up it's body, due to being more democratic in nature. Hard-line Islamist groups are sidelined, and threatened with military action, while the moderate forces achieve a huge boost. Turkey sides with the new federation, as their goals are similar, though they both still consider each others rivals, as they're both vying for influence within Syria.
On ISIS:
While Assad may have ignored ISIS activities, the withdrawal of Hezbullah and Iran caused ISIS to expand rapidly, though temporarily. Alarmed by this, the international coalition, with the permission of the federal air force enforcing a no-fly-zone starts an air campaign against ISIS targets in Syria. With a far more coordinated effort, ISIS loses far more territory to date, than our timeline (ours is 40-50%, making the alt timeline somewhere between 60-70%).
If eventually ISIS is defeated, whether in our timeline, or the alternate timeline, it won't matter if Iraq's internal political differences aren't addressed. The Gulf Federation, having pretty much the same goal as KSA, UAE...etc, will probably not help matters, and may even hinder any sort of progress in Iraq. Iran and the TGF will probably continue to fight it out through proxies within a post-ISIS Iraq and Syria.
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Anyway, I could go on and I really want to, but I don't have much times.
