Unsure if this has been suggested or not, but here’s a quick example:
I figured that, since defeating the Kraken a lot gives you kraken stuff for both your pirate and your ship, I figured that me defeating the Shrouded Ghost would’ve rewarded something really cool, but upon checking I got nothing which was kinda disappointing since the mythical Ghost is so so rare. The title itself was amazing to have but imagine being able to show off such a cool achievement through ship or pirate cosmetics. The same could go for the defeat of Ancient Terror, Crested Queen, Shadowmaw, and The Hungering One.
Just a small suggestion so we have more stuff to waste our gold on XD