So, what is the latest possible pod for a united Arab nation?
Iraq joins Nasser's UAR in 1957, tilting the balance of that union decisively in favor of the Nasserists both at home (preserving its stability) and in the Arab Cold War. Prince Talal rallies his Free Princes movement upon seeing the apparent success of the Nasserists in the late 50s and successfully launches a coup following the weakening of King Saud. Saudi Arabia joins soon after, and the Gulf monarchies are soon swept aside in a wave of popular revolutions much like 1848. Without Saudi support, Yemen falls to the UAR and officially joins in. Lebanon falls to a sponsored coup the moment US troops are unable to keep the lid on the sectarian mess bubbling up.
Finally in 1969, upon Jafar Nimeiry's coup in Sudan, his pro Arab nationalism stance and initial popularity lets him smoothly integrate the Sudanese republic with the burgenoning and blossoming UAR; the Sudanese people having growed increasingly apathetic to a decade of sluggish growth, isolation, and political instability.
Throw in a probably inevitable war with Israel that this massive superstate manages to conquer it via changing doctrine to mobilize intiative among ground troops, logistics, coordination, investing in better AA guns, and copious amounts of Soviet surplus and you've got everything united around 1970.
How populated would it be?
So the population would likely be less than what is historically the case; combination of greater wealth, Nasserist policies on education prior to Intifah, women's emancipation/education, and urbanization would mean a smaller fertility rate and therefore an earlier demographic transition. Entire thing might be around 200-250 million or so.
How powerful would it be?
Just by oil and manpower alone it would be a regional powerhouse or a great power.
This theoretical state houses I believe slightly over 30% of the world's proven oil reserves and would hold immense deposits of minerals (including aluminium, barite, basalt, gold, silica, marble, limestone, precious gems, etc). Additionally it would have an utterly massive domestic market of buyers that would be well linked and educated through its oil wealth and import substitution policies. Other important sectors include vast agriculture potential in grains, cotton, citrus; and and given Nasserist policies large amounts of heavy industry (your milage may vary on how efficient it'll be) alongside an emphasis on transport and shipping. Oh and of course, the inevitable service sector. Hard to estimate how that will go since it depends on so many factors.
How would minorities fare?
Difficult to determine. There likely will be be great pressure to assimilate or at least learn Arabic.
Could it become an industrial power with european levels of living standards and literacy?
Yes; the Nasserist policies of import substitution can do a lot to start from scratch given the huge domestic market for labor intensive manufacturing and the subsidies from oil paying for basically everything at first, up to and including a robust welfare state and quality education. The best part is education would get much more efficient over time as the increasingly educated classes are able to aid their children in school and in turn invest in education privately.
Could it have a lot of scientific output much like Germany does otl?
Would take a few decades to get the necessary critical mass of literati, accumulated business and technical knowledge, and R&D funding, but looking at China and Iran we have successful models of state led programs to dramatically expand research in just a few decades.
