I think the introduction of a few lighthouses to the map would be a very atmospheric addition to SoT.
A small stone lighthouse that would shine out across the sea in fog, storms or at night would be nice to see and could be useful for navigation (especially in fog or storms). Perhaps it could house a bell too for audible navigation in fog.
While these could be additional islets/structures, I think that replacing the seaposts with them makes a lot of sense. You wouldn't have to change the current seapost function, the hunter's call and the shops could still be there, but the current shop keepers could become lighthouse keepers as well. In the case of Sudds, his lighthouse could maybe be a bit more of an observatory, with a telescope or something.
Perhaps this could be a stand-alone thing or worked in to a hunter's call rework in future.
