tl;dr - Add Black (Swamp) cards. Swamp functions as an aquatic land.
We know that the hexagon has six sides. If we correlate each side with a color, we would get 6 different colors (or 5 if neutral is counted as a color). At the very least, it means one color is missing.
What I would like to suggest is to make this “missing color” black for Swamp.
The only tiles that aquatic followers can move into are Lake tiles. As of now, there are only five aquatic followers - too few to warrant the addition of a new aquatic tile. However! As more aquatic followers are developed, adding a new aquatic land into the power wheel would allow aquatic followers more freedom of movement across the board.
When paired with blue (Lake), I feel the emphasis should be more on board control and the stealth mechanic. I don’t know what the development team wants ‘stealth’ to be in Faeria, but I feel it should be land-based with mobility. For example, “this follower has Stealth while on a Swamp tile”. Or, “this follower has Stealth while on an Ocean tile”. On that note, when an opponent’s follower moves into a land occupied by an allied stealth follower, both will go into combat, and the stealth follower will lose its Stealth ability for the rest of the match or until it’s reapplied through an event. On an end note, I think the duo Black/Blue card should be an aquatic follower with stealth that can move stealthily on both lake and swamp tiles (until it attacks or goes into combat with an enemy follower that is attempting to move into its swamp/lake tile).
As for mono-Black, maybe gives us skeletons, zombies, specters, sea monsters, and skeletal dragons. Life Drain (from yellow) could go to Black.
That’s all I have for today. Thanks for reading!