Agreed, perhaps ports themselves could even be somewhat modular, not all ports are the same or as substantial and especially during this era in history. The game ought to reflect this.
The simplest and lamest implementation would be to create a few new buildings to represent different types of ports with different buffs
Ex- Military/Ship building Port, Trading Port, Storage/Supply (?) etc
Honestly the trade system is still too underdone for ports to actually function as they historically did. The easiest "issue" to highlight is how abstract exporting and importing goods really is. For example no mind is paid to HOW goods get to where they are going, I could be a landlocked nation in the middle of Germania, but as long as I'm within diplomatic distance I can export some stone to another landlocked nation in Britannia or to my neighbor for the SAME PRICE and quite literally ignore this route for the rest of the game, and if the route is cancelled it just gets taken by some nation in Iberia for the SAME PRICE centuries later.
How does the stone get there? What route do the merchants take? what about the hundreds of miles of savage woodland they must traverse? What about the hostile nation they must travel through? Why would a stone deprived nation pay me just the same for a nation with an abundance? Why dont prices change, ever?
The very fact that imperator has no answer for these questions is an issue. The game doesn't need to be vicky 2 to have a compelling trade system, but the problem is, the trade system really isn't compelling at all, just a necessity which they made as easy as possible with no substance or even RP value. Yeah I conquered all of Germania and have access to an unimaginable quantity of natural resources, just accept a few trade requests and watch my income increase by a few points a month, no substance.
For ports to matter, not only does there need to be some modularity and customization to the ports themselves, the way the game treats exporting, importing and resources in general needs to get away from this horribly simplistic and quite frankly boring system. The solution isn't a new system that's tedious and useless, I don't want Imperator to be a cumbersome logistics simulator, but to have the game actually give weight to the necessities and problems of sending horses halfway across the known world. Imagine being able to open up the trade map mode, click on a province and see the route the caravan is taking, see what ports they decide utilize, the risks they are taking, what cities they going towards, the risks and possible loss of profits.
Maybe you own the entire Mediterranean as Rome, but pirates are still plaguing your internal trade routes through certain patches of the sea, now those traders from the east who have travelled so far to reach Rome are getting picked off, not only creating diplomatic tension, but causing unrest at home and a dip in profits. Now you have to conduct an anti piracy campaign to protect your trade. In this way, ports would be concrete in nature, now I can't just import grain to an undeveloped, portless region and not pay mind to a dip in profits, or the lengthy trade route over land, which would facilitate me and the AI to build a port for traders to access the region, and maybe ill even expand the port so take in more goods. Maybe a stretch of coast is so lengthy and lacking in ports, traders avoid taking the route due to a concern for running out of supplies at sea, so instead they take a somewhat less direct route to stay safe before continuing on, causing -again- a slight dip in profit.
Ports and trade need to be fixed for sure
