the cemetary cannot be changed much cause the hidden mechanics secretly linked to it . you can change the look of the tombstones, you can change the look of the fences all around but you cannot add another ground, i mean a floor. you can change the texture on the ground, but you cannot add the floor itself, so the tombs would stand floating on the water. and you need to keep the way it is done, you cannot decide it is a crypt like, or a dock, or a building. you click and drag and it put fences all around and the game makes appear tombstone randomly inside.
i know when we are not a banished modder it can be difficult to know what we can do... what is a building/resources... and what is a game mechanic.
i hope the explanations of each part help you to understand what can be done.
