Well you are not describing actual gameplay or gamedesign. KSP is not really about flying rockets (I know it sounds weird) - it’s actually a sandbox puzzle game in it’s core. You are given a limited amount of tools to solve a puzzle (get to orbit for example) and by accomplishing it you get more complex puzzle (get to Mun) and more or better tools to solve it. From this perspective you can make the same game as KSP but with submarines, or cars and it will just attract different people, who prefer submarines or car setting to rockets. While you core gameplay will still be similar to KSP.
I don’t know how much it will help you but with a lot of good games it’s not about adding more, but having just enough features for gameplay to be coherent. The amount of planets and moons in KSP is just as it is not because devs don’t have time to build more, but just because more is not necessary. As it is, they are different enough in a sense of a challenge, adding more won’t add to gameplay unless you add more features.
So I would focus on what makes KSP a great game and not how to make the same but more of it.
On other note, I had a prototype for 2d space exploration game and what I found problematic is how to make ships fun to fly. It’s much easier to control it in 2D comparing to 3D and pretty fast you can easily pilot even poorly made ships. You just have a less play-field to make a mistake and learn from it. If you try to make controls more complicated or with more inertia (as realistic simulation would do) then you get into trouble that people who are interested in this kind of “hardcore” gameplay most likely not interested in 2D arcade looking game.
Good luck with your ideas! I think such game could be fun but I wouldn’t focus on KSP as inspiration too much.