Undead pirate ships coming from the ground and start raiding an random place. That would be amazing and it will make sense in the lore.
You sure like the pirate idea, it's very interesting indeed and I'm sure the class will be unique.
As for the event with the undead pirate ship, personally I think It's cool, and it can be done with a random spawn, but on a certain day. So that way the players will know on which day the undead pirates will come, bit won't know where and will have to search and tell each other ;)