Either there will be an end game state or not. If there is no end game state then the game will, as all other fighting games have done, slowly loose player base and then fade. Naval action has brought together both the FPS game AND the strategic game play of a 4x game. Brilliant! It does however need an end game, or failing that, a way to slow down conquest so that the end game is ever near but very hard to get to.
Most games have a mechanic integral to them that slows expansion down either by making that next step increasingly more and more expensive ( requiring some cost of in-game resources not just money ) or is level / achievement gated ( you have to have x things before you can add one more y ). This can be simulating the ever increasing cost of managing a wide flung empire across a vast area. Spain, of course, had the new world wealth to pay for its expansion until Charles V blew most of it on fighting the reformation, and even they didn't get the whole enchilada.
Slowing down expansion will also solve the bandwagon effect where players move en mass from one or more "losing" nations to the perceived winner. Naval Action works best when the pot is simmering gently and hostilities are gradually increasing as is the player tension.
How about a maintenance mechanic whereby the nation that has captured a county must kill some amount of BR of enemy in that county's zone of influence ( not its own and no Pirate Shenanigans ) each period and that this cost diminish over time to allow the nation to move on to other conquests? Perhaps these could be missions and allow newer players to contribute in lower rated ships ( humm lower rated ships higher number..bah! ) This would force captains out of the ale houses (internet cafes) and back into their ships in the ow and play to Naval Actions biggest strength the OW combat mini-game. The more counties the nation hold the higher initial maintenance cost of the newest county.