The concept of skeleton keys in Wizard101 was very well suited to that game, and I figured I should come up with something similar for pirates.

The first part of my idea involves a slight modification to the victory conditions mechanic. The flags at the top of the screen that show the victory conditions for a battle would come in different colors, each one indicating what type of goal it is.

Green flags are goals that will win the battle when they are met, blue flags advance to the next stage in the battle (think Gortez jumping off his throne) and red ones mean failure if their conditions are not met (protect Captain Gordon). Then there would be a new type of condition displayed on a yellow flag.

Goals on a yellow flag will not have any effect on whether you win the battle, and can be ignored completely if the player wants. They are secondary goals, sort of a challenge for an achievement in a battle, such as destroying certain obstacles or finishing the fight in a certain number of rounds.

Completing these yellow flag goals will win the player "pieces of eight" a special currency used to pay at the things in dungeons that are the equivalent to the wizards skeleton locks. I'm sort of thinking something like a tollbooth being run by a troll. I think the things with rewards and priced increasing with level go without saying.