Darius offered Alexander land west of Eupharates: Asia Minor, Egypt, and part of the Fertile Crescent.
I noticed that later empires with such territorial combination: Greece + Egypt + Asia Minor, such as Byzantine and Ottoman Empires, would generally be quite stable. Moreover without exhaustion during the expeditions, both Alexander and his army would be in a better shape consolidating the empire.
Would Alexander's Empire last longer if he accepted such an offer? And we would see a forerunner of OTL's Byzantine and Ottoman Empires?
Or would its internal strives make it collapse just like his empire did ?
(This is a "what if", not an AHC)