Hello.
I don't have any experience with making mods AT ALL, but i do have an idea. A mod that kinda adds a surface to Jool.
Its actually just clouds, but it acts like water.
It is not flat, just random clouds starting at a height of approximality of 1KM.
If you manage to have such a heavy ship or use engines, or just somehow sink until 100M, the clouds will stop, and you will fall out of them. and on 50M, an ocean appears.
This ocean is the last layer of Jool, and continues until the -250M mark, then your ship will, as always, explode on that height.
BUT there is even more! if you manage to somehow accelerate to 200m/s and go through -250M, you will appear on a tiny rock, 10KM the size, and is the core of Jool. it's small, but if you take a surface sample,you will get 7.000 science points.
Let me know what you think!