If the Greeks get free reign in the western Med' from the late 7th BCE onward, you'll see more colonies founded by refugees of the Persian conquest of Asia Minor (as it happens Phoceans were already founding places like Massalia, and when they were conquered at home they fled and founded Alalia in Corsica before a big fight against a combined etruscan and carthaginian fleet that ended in a tactical victory but strategic defeat that lead them to abandon Corsica and found Velia on the italian coast).
So we'll see more places like Empurias spring on southern France's coast as well as on the Spanish eastern coast, probably at least until Gibraltar. They dominate the trade despite attempts by the Etruscan to fight them.
Two main cities arise in this western greek sea, Syracusa and Massalia, who get into commercial fights with the other cities allied to one or the other (less fighting between cities in Italy, Gaul and Spain because of growth space and external threat of Gauls, Celtiberians and Italic tribes).
The Etruscan loose earlier their power in the Naples area, and Rome comes earlier in contact with the Greeks, with maybe a fight between pro-Greek and pro-Etruscan factions in Rome during the 5th BCE.
During the Persian Wars the western Greeks are able to send some forces to help against the Persians while those have less ships in their hands, leading to earlier and more successful campaigns against Rhodes, the islands and the coast of Asia Minor, maybe leading to an overconfident greek army fighting in Asia Minor instead of in Greece like it was for Platea, and being soundly beaten, leading to a status quo. In any case Athens does not rise as high as it did OTL, because the Athenian contribution to the fight is proportionally smaller and they may not be able to overtake the league due to Sicilian interference.
The Peloponesian war equivalent is much larger because this time the Ionians may count on the Massalians to help while Sparta and the Dorians get access to the Sicilian fleet much earlier, leading to a bigger challenge to Athens. Corcyra chooses earlier a side in which to engage its massive fleet (possibly the Athenian, to prevent Sicilian domination in the Adriatic and western greek areas)
Higher and earlier losses at sea for all participants lead to an earlier stalemate and possibly an earlier, more dynamic Sicilian expedition which the Athens-Corcyre-Massalia coalition wins. The larger war might also prevent a Nicias-peace like settlement because more parties have interest (instead of Athens vs Sparta it is Athens+Massalia vs Sparta+Syracusa). Also the higher tempo of naval warfare earlier on in the war might lessen or even butterfly the Athenian plague (less people crowding in the long walls because more people at sea).
In the end economy tilt the balance against Sparta and lead toward a division of the Greek world between Athens and Massalia. Thebe, not being able to develop in security from Athens, does not have its time in the sun. Sparta, due to its citizenship policy, is broken. Corinth is too close to Athens to be able to become a new leader for the Peloponese. Athens dominate through a league that gives a lot of importance to trade and fighting Persia.
The conflict give as many former soldiers ready to bear weapons for money as the OTL one and a 10 000-like expedition against the Persians, and then to the great war against Persia which is much more limited in scope than Alexander's expedition and attempt to give birth to independant countries carved from the Persian Empire, mostly city-states in Asia Minor up to Syria, a syrian polity and an independant Egypt with greek colonies for the trade.
Under the guidance of Athens, colonization in the Black Sea area is also much more dynamic, with more cities surrounding the whole sea and providing a stronger greek influence on the nomadic tribes of the plains of Ukraine and Georgia.
Rome, during this time, battles against the Etruscan and the Italic tribes, which will be weaker due to the stronger greek presence in the south. Greek cities do not fall into the ends of the italic tribes during the 4th and 3rd BCE, and Rome mostly sign alliance and trade pacts with them, wary of Massalia.
The Massalian federation of the western greek world probably founds colonies in Carthage and on the western north-african coast and looks toward the Atlantic trade system of the Celts for new ways to make profits. She relies much more on mercenaries than Athens, and always attempt to get new colonists from the egean world for her cities, with those angry of Athens' dominance taking the offer.
The great gallic migrations of the 4th century cause panic in Massalia and alliance with Rome, leading to a big fight in Cisalpina where the combined greek and roman armies triumph over the Celts who then go to the Balkans.
The much stronger Athenian alliance prevents the rise of Macedonia, which under the rule of its Philip II equivalent will look north toward Thracia and west toward Illyria both as a way to improve its security and to get new lands.
The 3rd BCE sees the big show down between Rome and Massalia. Rome, after the victory against the Gauls, has taken Cisalpin Gaul and looks to expand further but feels blocked by Massalia, which stands in the path of any western expansion and keeps control of Sicily, Sardinia and Corsica, dominating all trade.
Massalia, while powerful with her many celtic and greek mercenaries, is too close to the roman demographic jungernaut. Two wars will take place, with the first seeing the greek cities of Italy and Massalia itself become roman, the second with the exiled Massalians (reinstalled in Syracusa) loosing Sicily and control of their economic empire to a Roman invasion which culminate with the siege of Syracuse.
During this time, the long standing athenian league has become a more democratic hellenic league. Disputes between cities are settled in Athens' courts by tribunals made by elected representatives from all the cities of the league.
Currently the league is at war with Macedonia who has tried to take control of the Bosphorus after having consolidated its northern and western borders, but the ressources of the united greeks and their capacity to deploy forces on so many parts of the Macedonian kingdom, from the south, east and west, means that Macedonia is defeated and broken up in smaller city states which are included in the league, with league garisons put in place.
The later 3rd and the 2nd BCE see the Hellenic league develop new colonies in the agricultural plains of the Danube and of Ukraine, but also the resurgence of a new threat from the east. The smaller nations born from the break up of the Persian Empire have progressively fallen under the control of a new power, the Parthians. Born on the high plateaux of Iran, they have slowly beaten Mesopotamia to submission before attaking Syria and eastern Asia Minor.
(and that's all for now, folks !)