Hi,
I would like to suggest that after a next map reset / wipe all nations get the same amount of cities as a start, regardless of historical correctness. Just to give each nation the same starting chance and possibilities!
You could also definde victory conditions and when they are met a nation is declared the winner, then the map is reset and all start over again with a new campaign.
Or You just say a map/campaign runs lets say 2 months real life time and then a winner is declared counting the numbers of ports each nation has. After that there is a reset and a fresh start.
So every player can decide if they want to play for another nation in the next campaign. Clans and nations can try different strategies and tactics with a new campaign.
Just some ideas...