Alexander couldn't have made it to China. I'd say he could probably add the Arabian coasts to his empire, but beyond that, things become very difficult.
Plus, there'll be constant revolts. Alexander's attempts to plot a middle way between Graeco-Macedonian and Iranian customs tended to alienate both parties. His loyalists, of course, will stand by him, but there weren't too many die-hards by the time of his death. Outlying areas, whether under Iranian or Macedonian satraps, will be trying to break away for quite a while.