A Navy Card
Often, in warlight, attacks happen across bodies of water. In order to make it more realistic, you should only be able to attack across water if you have some kind of navy, possibly by obtaining some sort of navy card in some way. How I speculate a setting like this could be put into the game is that a new setting is added to map design, which would allow map creators to decide which connections would be maritime. Then, when creating a game, one could turn the card on or off. It would be entirely optional. This would certainly change the game a lot, but I believe it would be for a positive one.
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John commented
wouldnt that be marines?