Annie the Mermaid from the video game sequel is, in fact, Sloth.Some randomly visiting deity; possibly Haruhi Suzumiya, figured that there needed to be a love interest in the story; and that Sloth was the wrong gender and kinda ugly. One POOF later, and ta-dah!
Mikey seems to know more about One-Eyed Willy than he's telling.Mikey tells One-Eyed Willy he's the "first Goonie". Could it mean One-Eyed Willy is his ancestor? Was the surviving pirate who escaped the massacre with the map and the doubloon Willy's son?
Mikey is the reincarnation of One-Eyed WillyAn elaboration of the above theory; he seems to have an almost intuitive knowledge of One-Eyed Willy's treasure: he is able to find what professional treasure hunters couldn't, he successfully avoids the traps (especially the final one), and he even has a similar love of Rube Goldberg-esque devices.
Chester Copperpot is Mikey's grandfather or other relativeChester Copperpot was one of the only people to know about the treasure and look for it (even though he failed). Mikey finds a map and a metal or stone key in his attic. These items, but especially the skull-shaped key, are probably the only ones of their kind, so in order for Copperpot to get as far as he did, he would have needed at least the map. However, he didn't bring the map with him because he either memorized it (he was a dedicated old coot) or he found the shortcut through the well. Mikey has the map and the key because Copperpot is his ancestor, and his dad inherited it when Chester never came back. This would also explain the newspaper article in the attic.
The treasure map clues conveniently rhyme in English because they were written in EnglishThe Conveniently Precise Translation is possible because One-Eyed Willy (not exactly a Spanish-sounding name) wrote the clues in English originally, and then translated them into Spanish as an extra layer of obfuscation. Discovering that the English translation can easily be made to rhyme is a clue to the person using the map that they have the correct wording.
One-eyed Willy had that name long before losing an eye.Not that they'd mention or know it in the move, if you know what I mean.