One mechanism that would be handy for nations would be the ability to organize a national navy, primarily for combat. Under the order of the king, a nation should be able to provide general orders for the subjects of the nation in order to expand or defend holdings.
The concept is pretty simple - general orders are given by the king and parliament, and once passed, subjects of the nation enlist selected fleets to serve in the nation's navy.
The following general orders could be issued:
Blockade: This naval order would keep all naval traffic, or only certain nations, guilds, or individuals from entering or leaving a port. Once the order has been voted on and passed by parliament, subjects of the nation would enter the fleet number of the battle fleet they intend to place at the port to patrol. Once the fleet arrives, it would then begin to patrol the port to enforce the blockade. For every fleet placed into service for the blockade, the nation would pay for upkeep on the fleet (which would be double for war time), yet retain 10-50% of the bounty won in victory.
A blockade against a port would reduce the tax income of the NPC tax to 1/2, and influence would drop double (or triple, perhaps) to all those with influence in the port - unless they can get past the blockade.
Break the Blockade: The naval order would be to go forth and break the blockade. Subjects would again place the id number of the fleet in service, then proceed to the port that has been blockaded. If they make it through the blockade, they would begin a patrol to locate the enemy patrols, and prepare to fight them. (If they get past the blockade, influence would reduce at its normal rates.)
Run the Blockade: This order would be an attempt to run a fleet past the blockade. If it gets past the blockade, the influence in that port would be saved for the nation. Once the order passes the vote, subjects would place the id number of the fleet to go into service, then proceed to the blockaded fort and attempt to run the blockade.
Patrol: The order would be to attack and plunder the fleets of a certain nation, guild or individual. Once again, enter the fleet id of the battle fleet to go into service. This time, the fleet could be put into a trade route. When it arrives at port, using the gossip page, if a target is found, it will be attacked. If not, it moves onto the next port. Once again, the nation pays for upkeep (x2) and splits the bounty with the owner of the fleet.
These are just a few ideas to spur discussion and pursuit to move the national battles to sea and out of the parliament!