Sea shells and starfish would be nice. How you imagine it? Just as decoration? I'm thinking it could work as a harvestable stone resource. From the small gameplay test I had, resources (mainly clay and stone) are super hard to get (wood too, but after you have a tree farm those are easy).
Yes, 8bit crabs! No, serious, that would be so cool
The other biomes ideas are:
Sakura (all pink): this one is almost as complete as the archipelago, also lacking proper trees. (and a few other tweaks)
Volcano: worked good on my head, but super lame in-game (I think)... It is just a bunch of steep mountains with lava (orange water) at the top, and many small rocks scattered through the entire map.
And last my main, the reason I made all the others as exercise, the Swamp. I actually played a lot on this one already, and liked it a lot. But it also lacks customization, it is all vanilla assets. Terrain was almost the way I wanted, I can probably improve it now using a few changes that I learned and implemented on the others.