In my opinion, those holding a port should gain more from doing so, and I think that income should be linked to trade. I therefore suggest an increase in the trade tax given to the port controlling nation. I see two possible ways to do this:
A) Set the tax to 10% on all goods bought AND sold at port, paid by the game (like the current tax is).
B) Make those who buy and sell pay 10% in tax, but adjust selling and buying prices so merchants still earn the same after tax (I do not wish for merchants to suffer from this).
Alternative A is very easy to implement, and would work just like today, while alternative B has the advantages of being easy to expand in the future, for instance by letting the port controlling nation be able to adjust the tax within certain limits, and maybe it also "feels more correct".
While this would work great together with Blockades, I also believe this is a good idea even without Blockades, as I believe ports should give more income, and thus more reason to fight over port control, and also it gives people a higher incentive to trade in ports controlled by their own nation.