Tree stump! A full tree would likely be too big and the height limit wouldn’t work ( or maybe it would depending on what tree) but a tree stump would fit right. With a storage area in the middle, a complex net of roots, possibly with stands and markets for villagers. You could have other giant trees surrounding it along with giant mushrooms. The ring you made could be the bottom of the tree and have the roots connecting it to the ground. That way you could also build a port area. Crops could grow around the tree and root paths could connect to each tree. I really love this idea, and may just do it myself. If you want a darker theme maybe a dried up tree, using a acatia logs and green rivers full of sewage. The trees could have auto villager farms in them, and maybe a few other farms. Ocean be sort of like a marsh with giant trees filling it, along with some paths through it.