So, assuming the Hunter's Call ever gets some kind of Emissary situation, this is what I'd suggest for the voyage you could get once you hit grade 5:
It'd basically be a recycled Lost Shipment voyage, but instead of trying to track down a sunken Merchant vessel, it'd lead to a special shipwreck of a Hunter's Call ship chock full of fish in the its barrels. Instead of normal loot that might be found in a shipwreck, perhaps the spots where loot would spawn could instead mostly be Treacherous Plunder junk items with a chance of some occasional Mermaid Gems scattered throughout. Upon tracking down the clues and sailing up to the shipwreck, the Kraken could be a guaranteed spawn. This could give the idea that the Kraken is what sank the missing Hunter's Call ship that was tracked down and the damaging ink that comes with the Kraken could perhaps act as a deterrent from just diving down and grabbing the fish from the Shipwreck for an easy payout without fighting it. Plus you could get some guaranteed Kraken Meat to sell alongside it's regular loot (unless it dropped only Mermaid Gems or something else or no extra loot at all).
I originally thought maybe it could be a chance between the Kraken and a random Megalodon (minus the shrouded ghost), but I think guaranteeing just the Kraken spawning in order to use the ink's damage (plus making the boat get stuck) to discourage just grabbing the fish and going was a good a idea. Perhaps Megalodon hunts could be instead implemented as a normal buyable Hunter's Call voyage that could incorporate playing "Summon the Megalodon" on a drum in some way to harken back to Hungering Deep with glowing tattoos and all when it's summoned, but perhaps the song could just be played at whatever place the Megalodon would be summoned instead of having the carry the tune all the way from Sharkbait Cove. And maybe this voyage could involve tracking it similar to the idea for the Emissary Voyage, but perhaps without the shipwreck at the end.
Thoughts?
