Speculation: I think the Freak is going to be Fred's dad, the Mayor. Why else would he want to warn the original Mystery Inc. and Scooby and the gang that "the curse had begun again" and play blissfully ignorant with the gang in his search for the treasure?
Speculation: The gang will capture Pericles but the victory will be short-lived. There's something near extra-terrestrial about him, so he'll escape, perhaps with the treasure thus denying Fred's dad and Mr. E their spoils.
Speculation (and God have mercy on me if I'm right): The whole thing is either a dream Fred had or an extensively written first-person fiction by either Daphne or Velma.