This might not count, but I would suggest splato (Dalmatia), for
https://en.wikipedia.org/wiki/Diocletian%27s_Palace. It was a pretty safe location to conduct trade around, and if the aqueduct was restored, splato could have grown into a proper merchant city (instead of a Venetian colony as otl)
In actual Italy, I suggest Ravenna (its port would have to be re-dredged), Naples (going merchant route instead of kingdom route), Brisindi, Taranto, Aquelia (old roman city that could have been rebuilt, but wasn't), and Reggio de calabaria.
In the Mediterranean outside Italy I suggest Marseilles, Barcelona, Valencia, Palermo, Tunis (could be settled by Europeans, the Spanish did control it for a long time otl but did very little with it), Dyrrachium[durres] (merchant republic of eprius for the WIN), Thessaloniki (would need kickstarting, probably started by crusaders but continued after they leave?), Antioch (same as Thessaloniki), and Alexandria (same as Thessaloniki).
Many of these are probably asb, but I threw everything I had into there, so you could probably make use of something

. Most of the merchant republics controlled very little outside their main city, so I don't think I need to explain what they control. (but if you do want me to, feel free to ask)
I challenge someone to make a tl off of this somehow

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