What if Alexander not only lived longer, but tried again in India? How long would he have to wait to gather a large enough army? He was young enough (32) at his death that he could wait a whole generation if necessary, and still would have been able to lead troops.
It would be more like having to create a whole new army, since the troops he had were his fathers army.
Literally his father army, ie the same men his father recruited when he was a young king, and therefore some of them were grandfathers by the time they reached India. That is one big reason they demanded to go home, men in their 50's and 60's made up the majority of many units.
So the army Alexander would have with him would be recruited from his new empire, and he actually WAS creating a new Hellenistic army of Persian recruits. His Macedonian troops were insulted when the first regiment of them turned up, but any new war would be fought by Persians and others trained in the Macedonian way of war.
Alexander therefore would not have to wait a generation, since he already had new regiments arriving and old ones leaving to go home to Macedonia and see their grandchildren. He and his senior officers - the guys he had gone to school with - had married Persians as part of his plan to create a coherent empire. Continuing his conquests after a wait for enough new regiments to replace the old ones leaving would help with his long term stabilisation plan.
Continued expansion would mean the newly conquored areas providing the resources to support new wars. To 'make war feed war' as it is sometimes called, and india was wealthy and populous enough to do that. So if Alexander does not die after reaching India but pauses, renews his army with new regiments of Imperial citizens replacing the old Macedonians going home, he and his officers Persian wives giving them families - instead of being instantly divorced/murdered at the start of the civil war fought by his generals after Alexanders death - we have an entirely new world.
A world where Alexander the Great's empire maybe actually survived him rather then his generals responding to his death by hunting down his wife and son, murdering them and then declaring war on each other. That would be a different world with butterflies the size of Mothra.