Alexander lived to 80 years old... what changes?

[Frodo]

I think after getting reinforcements, he would have tried to conquer Arabia for sure, which would most certainly had happened had he not died so soon.  And then turned his attention once again to India given he had unfinished business from the last time before his armies forced him to withdraw, with the intention of consolidating his Indus river valley conquests.  Which would bring him and Chandragupta Maurya face-to-face on the battlefield.    

[Frodo]

Also, in addition to what I said earlier, if Alexander had lived longer, he would have had time to consolidate and institutionalize his goal of syncretizing Hellenistic and native cultures with his commanders marrying and having families with the local nobility (particularly the Persians), and founding his own Heraclid dynasty that would outlast him while ruling a vast and united empire (with its capital at Babylon) stretching from Egypt and Macedon to Bactria and India.  Or at least the Indus river valley portion of the subcontinent.  

And perhaps one of his successors would conquer northern India itself sometime in the future, with the area conquered approximating that won by the Kushans in OTL.  

And given a couple of centuries, the empire would derive much of its revenue from the Silk Road (both the overland and sea routes), further filling its treasury for future conquests.