A union between say Syria, Jordan, Hatay (Antakiyya), Palestine, Iraq, etc might be more plausible. The issue is, when you add all of Arabia, Egypt, North Africa, parts of Iran, etc... Somalia, Afar, Sudan, etc is simply too much, these areas were not even part of the Abbasid or Umayyad caliphate are not Arab in any serious sense.
Iraq comes with many issues, it is divided religiously, ethnically and economically. Most of the southern reaches are heavily Shi’a and these in some areas are so dense that Sunni rule is accepted only through repression and or with amounts of autonomy or the historical usage of taqiyyah. The upper Euphrates especially, along Karbala was untenable for even unity to the Sunni areas of Anbar frankly and is distinct from Bagdad on the Tigris. This whole area would in the past, much prefer the rule of a legitimate Shi’a monarch from Iraq than be united by Arabism. The people in this region bemoaned for centuries Ottoman rule and the loss of the Safavid monarchy. Many Twelver Shi’a scholars in that area historically also, are not too charitable to the Arab ethnicity either, as the work al-Kafi stated, ‘the Arabs are the most vile folk, they murdered the prophet and rejected the Imamate’.
Iraq also possesses large amounts of Kurds, both Sunni, Shi’a and Yazidi who live throughout its territories. These Kurds had existed within relative decentralism for approximately 400 years. Forcing them into a pan-Arab state is going to make what the Turks have in their Kurdish areas seem easy. They will resist, radicalize and fight for every inch they can. The added dynamic of minorities among them complicated issues too, in 1900 a larger percentages of these areas are both Yazidi, Christian and Jewish. All of whom either survived as distinct due to high level autonomy given by Muslim states or by way of paying jizya tax. In this secular Arab state, their very existence is at stake more than ever.
Syria has much the same issues, as with the Levant. These areas though majority Sunni and Arab, have significant Christian presence, Druze, Twelvers and Alawites who all present difficulties. Kurds also, are united into bloccs with their neighbor cousins and likely have some sort of agreement with other minorities like the overarching Shi’a.
Egypt is too populous and ethnically-economically United, they will dominate the interests of this country and ruin and pretense at fairness in terms of election.
Arabia has so many tribes, Bedouin, divergent Islamic beliefs and disputes to be a positive addition. Yemen, Nejd, Ahsa, Oman and so forth are all divided religiously and to a degree ethnically. Yemen and Oman for instance, has villages who do not speak Arabic to any degree. In otl, these areas are often afforded a level of autonomy and liberty and attend to their tribal customs as they had always done. In the Nejd, too many Bedouin and Arabs will refuse public education or progressive political narratives.
Maghreb, Algeria, Chad, Libya, Tunisia, etc. Much of these peoples are not Arabs and many will not speak Arab without education. The Tuareg, Fulani, Berber, etc for instance do not speak Arabic, especially in rural areas. All of these areas too, are less economically developed and economically distinct to everywhere else in the union.
Somalia and the sectors mentioned of Ethiopia, were never ruled by the Abbasid or Umayyad and I do not know what to say for them.
Overall, my view is that the Ottomans could rule all of these areas aside from Somalia mostly. As could many states that utilize Islam and high levels of decentralism as is stipulated by traditional Islamic statecraft. However, the large centralized Arab secular state will have difficulties. Even in just the localized varieties of these states in Syria, Egypt, Libya, Iraq, etc, all we have is disaster and issues.
