I suggest that one new type of water body is implemented: Rivers. No new mechanics, just flavor. Perhaps a bridge rail if you feel like it. I however suggest the limiting factor of there cannot be more than one river per x number of tiles, and possibly have multiple rivers merge at points if that seems like a good idea.
I think this will add to the value of Factorio because it would give the game's terrain a little more variety and flavor without changing anything major, and I think many players would find rivers not only a strategic benefit due to high defense capability (Unless you make all the water shallow
), but a refreshing sight and an interesting land feature to build around, as I find the most beautiful and defensible structures in real life are the ones that incorporate the terrain. Look at Mont St. Michel, the castle in the middle of the sea, it incorporates the low lying terrain surrounding it into both it's defensive capability and the beauty of the design.