It's likely that Alexander would have taken over Italy had he lived a couple more decades. He definitely could have done it. Rome was not yet the powerhouse it became and he knew how to lead an army better than literally anyone in history. There were many Greek colonies in Italy but the whole peninsular might have become dominated by Greeks. By the time Rome got around to taking over Greece it wasn't even a contest.…
Italy was a back water in 300 bc. with the great political and cultural centres to the east of Greece. What would attract Alexander to Italy?