Now, I (I am not the creator of this wiki) theorize that the three unreleased sets (skeleton crew prison, marauders mutiny and privateers attack) was going to be the third line of skulls. Now just think about this, because I have proof: They all appeared at the same time, (albiet never released) just like the other ones. they also appear in a group of 3, which is a nice, round number and the same amount of islands in the original series. And on the latest map, (the one that has these 3 on the map) They have the islands all have skulls on them. Now, what other islands on the map have skulls? only 5, the original 3 skull caves and the 2 in the second line!
and furthermore, they are all clumped together in a single area. What else were all clumped together? the previous skull islands. I think I have given evidence to believe these 3 unreleased sets were to be skull caves!