@Wolttaire
Massalia, as a political power, was essentially reduced to a coastal presence that, in its apogee in the Vth century, went as far as Avignon and the Rhone's delta, supported by settlements but critically by integrated/clientelized oppidae and Gallic peoples. Due to the general crisis in the end of First Iron Age that I roughly described you
there, (basically trade road shifts and Halstattian initial changes), this presence was even more pushed back to the coast, being understood that Massalian colonies were largely independent and tied mostly trough religious and economical links.
Even before Hannibal's passage in pre-Alps, Massalia's chorê was extremely reduced and threatened by its neighbors, and it expanded significantly on the hinterland only after Roman intervention in Transalpina.
At the very least, you'd need a political dominance of Massalia upon its colonies, which would be easier said than done.
Massalia, possibly aided by an influx of Roman refugees, can expand into Gallia Cisalpina and/or northwards into Transalpina without worrying about being seen as a threat to Rome,
Massalia mostly avoided being eventually taken over by Celto-Ligurian confederacies thanks to Roman alliance. Without this, their relative hostility (mostly from Salues, and especially from the IInd century) would eventually take its lot. Maybe not taking the city itself (while it's nothing impossible in the right circumstances and the build up of a regional coalition), but possibly some of its colonies and even more of its remaining chora, to the point being probably as reduced as Emporion's.
and becomes a trade empire to rival Carthage, albeit trading more in raw materials including slaves on the Rhone and other rivers in the region.
In all likeness, slave trade really became a Gallic export only trough Roman presence and colonialism : grain and metals were generally the main trade goods brought by Greeks and Italians until the IInd century.
Note that the Rhodanian axis relatively declined from the Vth century onward, with the "Gallic isthmus" (roughly the Aude/Garonne axis) taking more importance in the West, and the Danubian/Rhine axis in the East.
Ultimately, they slowly Hellenize the Gauls into a variety of similar oligarchial republics,
IOTL, Massalian presence did introduced various hellenized features (such as alphabetization, sophistication of oppidae, etc.) which played against it : the confederation led by Salues was powerful enough to take on the city in the IInd century.
Long story short, while you could see a tyranny emerging in Massalia by the IInd century as a reaction against Celto-Ligurian threat, as a permanent state of emergency if you will, the relatively declining role of Massalia into Gallic trade (eastern Languedoc's oppidae and emporioi took on themselves to drain a significant part of Italian trade)
Even, because reasons, the confederation of Salues doesn't appear ITTL, there was a strong tendency to relative unification among Celto-Ligurians that would benefit a relatively further center (Voconces or Cavares, if not Allobroges due to their tradition of mercenariate in Cisalpina which would remain intact ITTL) .