IMHO it should be possible (maybe not easy

) to avoid the encroachment of northern France, both in political and linguistic terms, and have an effective division of "Francia" along an east-west boundary. This might come out of aresurgent kingdom of Arles (in the HRE or not) or by a more successful Toulouse. In either case I doubt that Marseilles would become the center of this Occitan commonwealth: the city lacks natural defenses,and its population has been always quite limited (IMHO the former is much more significant than the latter).
But Marseille as the center of Occitania is hardly Marseille as center of France.
The region Laon/Paris/Metz is simply too prosperous and too important politically and military to get rid of without an earlier POD.
LS Catilina and I discussed why Marseilles never matched the Italian seafaring republics not long ago. The presence of the Saracens was certainly a major obstacle, same as the depopulation of the coastal holdings. However the main reason for the "stunted" growth of Marseilles in the 10th-12th century was quite likely the lack of natural defenses landward.
This is less damageable in my opinion. After all, hellenic Massalia was an important city-state in the same context. And in late merovingian period, the town seems to have been important, if secondary comparated to Rose/Rhone cities. I mean, the archeological evidences shows a prosper city that decline in the VIII century.
But with the silting-up of Arles, and without both Islamic piracy and the Carolingian raids...A patrician Provence could have protected the city from the countryside, just like in the Antiquity.
The main problem was that : the decline of trade, the destructions made by Charles Martel, the Carolingian civil wars...That forced the nobles and their armies to be more focused on the passes and roads between Italy and Aquitaine, and therefore to focus more on land than sea.
And therefore, for the greater part of MA, the struggles of nobility to maintain their interest in Italy and critically in Aquitaine (with Marseilles being excommunicated by the pope to prefer Tolosa as suzerain) slowed the growth of the city who was still an important harbour.
But the harbour was mainly prosper between the 1000 and 1250, when the nobles weren't that busy with seafare and didn't cared that much to protect a too independent city.
And after that, ravaged by the catalans, by the plague, by the quasi-monopole of Italian cities...
So the city was too secondary for nobles since the 1000, too independent to be kindly protected and not powerful enough to get really independent (the Counts of Barcelona/Provence then the french kings weren't the Kaisers)