Venice is a little cliche now though. 

If I were you, this is what I would do. Since we already have sea merchant states, I would go a different direction. Maybe a "He who control's the spice control's the universe" situation where you have a nation or city state sitting on a rare, and very popular, resource. It has to be something that can make a city rich, while being impossible to find anywhere else. Luckily, this is fantasy, so you have options. Perhaps it is a naturally occurring drug that sprouts from the earth, like liquid LSD or some shit, that is incredibly valuable amongst the spiritual. Maybe it is something more mineral, like a type of glass that bubbles up from the ground and, when tempered a certain way, becomes as hard as steel. Maybe it is a full blow apocalypse-making crater so rich with meteorite stone that your city grew rich on it. Either way, the point is that you have a small state with a lot of money. This means merchants could become richer then land lords. They don't own companies or buy patches of land. Banking becomes a big business for them, since land-bound caravans would want binders for insurance and the ability to transfer money back and forth, since there would always be a risk of being robbed along the road. Other men pull their money by milking all the people in town with money in their pocket. And others might use illegal means, like selling goods at a discount price and then paying highwaymen to steal the goods back for them.

Banking families would rule the state, since they have the most money and the most invested in foreign affairs. These banking families have cohorts in every trade port they can get their hands on, and they have the ability to manipulate politics based on their interests by controlling what foreigners get loans. They might even give gifts to particularly useful allies.