To put it simply, mermaids have vast potential in sea of thieves and should be utilized more. While we have the benevolent ones that return us to our ships after a failed sword lunge, we have an absolute lack of ones of mischievous nature.
I think a wonderful content addition to the game would be sirens that cause a loss of control of ones ship near rocks and other dangers. There is so much real world pirate lore surrounding merfolk that it could turn out to be a true treasure trove of content. Imagine expoloring mermaid cities and ruins in search of stolen bounties from ships they have sunk. Rare, I beg you to steal this idea from me as I feel it would open up the most unexplored area of the sea of thieves, beneath the waves themself.
