I agree with sea exploration. There needs to be a way to attach either sails or have manual rows on your raft to explore different areas across the sea. When you explore new areas you can find other computer generated ships and rafts with NPC's on it where you can trade, fight, steal and whatnot, creating friendly or hostile relationships with other people. When you go to other areas, you would encounter different things flowing in the water, and in different directions. At endgame, if you have the resources and you have made generators from harnessing fire or using wave power, you could attach simple motors to your raft to get to areas faster. Since you and NPC's will be on the same ocean, there should be different weapons instead of a spear. To plunder other ships and sink them, you would need simple firearms, using rare gunpowder, scrap metal and other materials to make them. Or you could mount cannons which take up lots of space, are hard to maneuver and are expensive to fire and to craft, but that deal significant damage to other ships and NPC's.