[Closed gun ports]

Yes, open world models.

[Make skills attached to levelling]

Skills that can vary and reflect a captain's style and effect; such as choosing between more accuracy, faster reload, better sailing, more precise navigation, etc.

[Map suggestions]

To add immersion, make the map look like an 18th century map.  Also, allow for the player location to be updated daily, at noon, as that is when they would take their readings with their sextant.  Also, (see my other suggestion), make navigation a skill you can level that will result in faster OS time and more exact positioning on the map.

[Closed gun ports]

Show closed gun ports on OS models.

[Teleport suggestions]

To alleviate tedious OS sailing, allow people to teleport from outpost to outpost every 8? hours.  But to allow for interception and pvp, allow teleport only some specified distance from port to a random location near the port of target.  That would prevent safe teleporting from port to port.