Not Karnath related but wouldn't it be cool if they implemented a more robust or complete custom card game similar to the likes of Caravan and how that game worked in Fallout: New Vegas? Or even something more trading card game like Gwent.
A trading card game where part of the reason to add it in Sea of Thieves is to give players something to trade amongst each other or purchase with in-game gold the different cards. You could buy specific cards from specific NPC vendors meaning you would have to visit the specific Outpost or Island to get that card you want that fits your deck. Or even finding the cards as lootable objects in the world at random places? They could fit this into their live events too where they release a special rare card only during a weekend live event or something. Play during that event, do the voyage, and as a reward you get gold + the unique card.
to bad standard deck of card will most likely mean no hearthstone/Gwent like trading card game.
Finding and Buying cards then players could access their collection and start building your own deck. With varying strategies to deck building, keep it simple enough but complex enough to be engaging and give reason to find/purchase the rare and better cards.
This would actually be a really cool addition to the game world I think. Give the players who max out everything something to work towards as a nice side thing. Making you find them is a great way of implementing it to the world and promote exploring and searching islands to buy from specific vendors only is also a cool idea.
Of course this is completely skipable so casual players could pick them up if they happened to spot them with the sparkly thing like messages in a bottle or not.