The Greek poleis were fiercely independent. You could have unity imposed from the outside, by Macedon or Persia or Rome, certainly. Failing that, it's tough. A sufficient outside threat (like Persia) iOTL brought Greece together TEMPORARILY, but then they fell back squabbling again.
I think the best bet is probably the Delian League. If Athens can be kept from turning it into an Athenian Empire (as they did iOTL), it could be the nucleus for a united Greece. OTOH, I'm not at all sure of any way to keep Athens under control, so that, too, is tough.
If the Delian League can be held together as a federation, probably under successive external threats (Macedon, Epirus, Rome, Persia, what have you), maybe conquering Sparta (who isn't likely to want to join voluntarily)...
Maybe