Alexander's campaign plans after he returned from India mostly centered on the Persian Gulf and Dilmun (Bahrein) was scouted by one of his generals. Using Dilmun as a base, I can see Alexander going on to take the rest of the Persian Gulf coast and establish shipping routes with India, but east Africa and the trading cities of western Arabia, including Mecca, were hardly known by any Mediterranean peoples. Any exploration of east Africa wouldn't show any promising signs since Axum, the first Ethopian kingdom, emerged in the 2nd century AD, about 400 years later. Macedonian exploration from the Persian Gulf could reveal the Sabaen Kingdom in modern-day Yemen. The Sabaens were highly developed technically as proven by the remains of a great dam at their capitol of Marib that was the center of a large irrigation system. If Alexander is still alive at that point, I can see him conquering Saba (or Sheba), preserving the irrigation system there and allowing the region to remain an economic and cultural center that could very well go on to vassalize the cities in the Hedjaz as well as Dilmun and the Persian Gulf ports Alexander captured after his empire falls apart. Later, this stronger Saba could go on to conquer the Ethiopians and become the major naval power of the Indian Ocean.
When the Romans come along, they'd have a difficult time going on to conquer Saba since they'd have to build a fleet in the Red Sea from Egypt to defeat the Sabaen fleet, which is unlikely. Other than that, they could try to march across Arabia but it's unlikely any Roman army would be worth much after such a march when it reaches Medina. Rome and Saba would likely have a stalemate much like Rome had with the Parthians.
When, or if, Christianity arrives, it's possible Saba might convert eventually which could prevent the rise of Islam. Or they'd possibly convert to Hinduism or Buddhism before that with contacts with the kingdoms of India. Saba might go on to build trading posts along the eastern coastline of Africa and take Zanzibar and possibly Madagascar for themselves. Without the Arab invasions, the Middle East could see a three way warring and diplomacy between Saba, Byzantium and Persia. And that's about as far as I'll project that.
When the Romans come along, they'd have a difficult time going on to conquer Saba since they'd have to build a fleet in the Red Sea from Egypt to defeat the Sabaen fleet, which is unlikely. Other than that, they could try to march across Arabia but it's unlikely any Roman army would be worth much after such a march when it reaches Medina. Rome and Saba would likely have a stalemate much like Rome had with the Parthians.
When, or if, Christianity arrives, it's possible Saba might convert eventually which could prevent the rise of Islam. Or they'd possibly convert to Hinduism or Buddhism before that with contacts with the kingdoms of India. Saba might go on to build trading posts along the eastern coastline of Africa and take Zanzibar and possibly Madagascar for themselves. Without the Arab invasions, the Middle East could see a three way warring and diplomacy between Saba, Byzantium and Persia. And that's about as far as I'll project that.