This is something that I think would be used very often by everyone in the game; if we were able to create bridges it would save us the headache of having to swim or use a boat every time we want to cross a body of water. Of course it would not be simple to build them, I was thinking perhaps it should take 25 boards per tile the bridge has to cross. So if you were building it across a river 10 tiles wide it would take 250 boards to build. All of the bridges would have an opening under them so people can freely pass under them while on boats or swimming. Animals would also be able to cross them. I would also suggest that you can't build a bridge within 15 tiles of another bridge so that people can't cover all the water with them.
Also as a separate idea for bridges, perhaps we could build 3-4 tile bridges over cliffs, and their cost would be 75 boards per tile.