It would be nice if land/sea units could interact more:

1. The "attack" land units (catapult/canon/artillery/howitzer) should be able to attack naval units next to shore; e.g., a canon should be able to attack a galeon on a adjacent tile.
2. The naval units should be able to "bombard" undefended cities. Specifically, the attack of a naval (or air) unit on a city without any units in it is played out as if a "virtual settler" were built in the city (decreasing city size by 1) and the settler were attacked. If the attacker won, the city size is reduced by 1 (and the city is razed if its size were already 1). If the attacker lost, the city size is unchanged but its food stores are reduced in proportion to the damage sustained by the "virtual settler",

I think this would keep the balance between land/sea units while allowing for me interaction between them.