Not going to happen. Completly opposite Alexander's personality type. Sure, I can see him marching to Axum, under incredibly stretched circumstances. But that would cost him much of his army, and, considering this would probably happen after Egypt, would leave Persia on his lines of retreat. But even if he reaches Axum, his men will mutiny, and force him to return home, just as they did in India, possibly before he finishes the conquest. But let's assume he somehow gets there, and conquers Axum. he builds a fleet, and sails up the red sea, conquering everything in his path, probably butterflying away Islam. Why? Because he was moved by dreams of Greatness. Every one of his conquests was heading off towards a new, exotic, rich land. Not into untamed wilderness. Meanwhile, if his soldiers mutinied in India, they will mutiny if ordered to march into an endless continent. FInally, if Alexander is off in the heart of Africa, who exactly is keeping the empire in line?