Speaking hypothetically;
I find the idea of the ghost ship that mysteriously appears at a random time, draws a few people onboard then whisks them away, not to be seen for a day or something, quite interesting. Obviously an idea like this has potential, but it's far from usable. I'm not sure loot/exp rewards would work in the first place, considering
A) Level difference of players, it would really suck to be too high/too low level when that ship sailed in.
B) Number of players & group organisation, could vary wildly - see C.
C) I feel that the mystique of such an idea is that it's random, nobody knows much about it, and it raises interest in something and promotes people to take it upon themselves to 'explore'. It wouldn't work if people knew it was coming, you'd get a hardcore group bullying loners/other groups out of the way for the sake of loot or experience.
Players could board this random & rare ghost ship and be taken away to somewhere quiet for a full day in-game, during which the people on the ship are taken slowly through a story, such as the past of the previous crew and their fate, why the ship's haunted, why it's docking in random harbours, and what the ship wants with them. Think that could have priceless immersion value.