I have to say, fluid dynamics would be an amazing update. Not just clouds and running water, but lakes, or maybe even ice that can melt. Flowing lava, volcanoes, Ash plumes...
All of that was really cool, but I think it's important to recognize what people asking for ship Interiors really want; they want a more seamless experience connecting the ship parts of the game and the ground parts of the game.
And this is a reasonable complaint. It's a complaint that has been valid ever since Horizons came out; the vast majority of players barely do any SRV content, and when they choose to participate in Odyssey content, it tends to be a focused engagement, without much participation in space content, until they get bored with the ground content again.
Part of the problem is, the game seems very dedicated to keeping the two games separate. For example, the way you need to have a prolonged docking process before you can get out of your ship. Realistic? Perhaps. But much like Interstellar travel, sometimes it's important to sacrifice realism for the sake of player experience. Personally, I would be more than happy to sacrifice the ship's ability to land without a docking computer, if it meant I could drop off my SRV from midair, and later pick it up via Skyhook.
And, in my opinion, that's the sort of content to the game really needs; ways of bringing the two different games together in a player friendly manner. And, I think, what players are subconsciously craving when they ask for ship Interiors.