My vote: Enforce game rule coalitions. Reasoning: too many nations at war with each other. The current situation is too chaotic. Enforce game rule coalitions is just one of the solutions.
Alternative solution is too reduce the total amount of playable nations, as many people above already suggested.
My point of view: remove all the impossible nations (Russian Empire, Prussia and Polish Commonwealth). This will reduce the total amount of playable nations from eleven (11) to eight (8).
Not sure about Denmark and Sverige as a nation. Three (3) options:
You could keep them as a separate nation (current situation), keeping eight (8) playable nations.
You could merge them into one (1) nation (for example called Nordic or Scandinavian, where Russian Empire could fit into the 'Nordic' description, but obvious not Scandinavian :-) ).
You could remove both nations from the map, reducing playable nations to six (6).
To keep your game historically accurate, please create/make Elite NPC of the nations you decide to remove. In this way, these nations keep some form of 'presence'. If you decide to remove Denmark and Sverige (going for option three (3)), they should have more Elite NPC's than the impossible nations. Besides Elite NPC, you can also keep spawning regular NPC's of these removed nations from freetowns.
Another point about population: you can enforce game rule population. In the playable nation select screen, you can remove the option to join the nation(s) with the most average active players, calculated with hours active a day. For the sake of the gameplay and the historical accuracy, the treshhold percentage (%) for closing certain nations to join should be variabel to their historically presence.
For example: Great Britain and Spain should have a higher treshhold percentage (%) than Dutch and a way higher treshhold percentage (%) than Denmark and Sverige if you decide to keep them in the game. Assuming you remove the impossible nations, it could be something like this:
Current populations - Treshhold percentage (%) of server population
Great Britain 26.86% - 25% (should be highest due to most famous Navy in history and also a new player 'easy' nation)
Spain 8.83% - 20%
France 10.21% - 15%
Pirates 14.52% - 15%
Dutch 4.88% - 10%
United States 9.15% - 10%
Denmark 2.87% - 5%
Sverige 8.76% - 5%
Russian Empire 9.33% - Remove from game
Prussia 3.61% - Remove from game
Polish Commonwealth 0.97% - Remove from game
- Total 105%