This has been a topic for a while, and people will argue for days about how to solve it. So I'd like to give my solution to it.
Make it so the more ports a nation owns, the more influence they lose in those ports per day.
Such as, a person with a nation without a port loses 1% of his/her influence per day (standard)
A person with a nation that owns 1 ports loses 2% of his/her influence per day.
A person with a nation that owns 2 ports loses 3% of his/her influence per day.
More ports = multiplier
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This would
1. Punish nations that try to take a majority of the map.
2. Make people hesitate about taking ports.
3. Increase diversity, some nations may think that some ports are not worth the extra influence taken off while other nations with no ports see it as worth it.