While the current water is great for outdoor scenes, it can't be used in rooms. A very simple way to provide indoor liquids is to add them as floor tiles, This allows creating aqueducts, sewers, and flooded areas in dungeons. These floor tiles don't have to be fancy, just easily distinguishable from normal floor tiles. Blue for water, green for sewage, red for lava.
I would love that. I'm trying to make a flooded ruin, but the grid is at "floor level" floating in the sky. It makes it difficult to plan out a battle level for my players. Water tiles would be great, as well as being able to have water at a depth without the massive drop.
I would love to also have floor tiles that match the default grass that first populates on maps. For example, if I want to just create a fence I have to build a room but the floor in the room looks different than the default grass round it.
The option to have floor tiles that appear to be a pool of water so you can have submerged rooms in your dungeons, caves, etc.
As this is a mixed bag of suggestions you might want to list "X as room/floor tiles" on the specific stretch goal theme lists as well. I've included a few below as starters.