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If I knew how to draw and program, I would add air instead of the sea. For example, heroes with only flying units in their army could "take off" above the map and cross obstacles. Taking off and landing would cost a certain number of movement points, say 200. Spending the night in the air would cost even more, 500 movement points from the next day's turn. But what an intense game it would be, with aerial reconnaissance and battles in the sky. :)