There is already "gameedit" which allows for a lot of realtime level development. It's pretty close to being released offcially from what I can see.

There is work on a menu editor. I don't know what it's state of development is but it's in the works by a forum member

As far as your other suggestions, a physics, sky, and lensflare creator make no sense. Physics is a purely programming application ad sky and lensflare are just textures.

I have no idea what a cutscene creator would look like... maybe like MS moviemaker? But that would only be useful if you wanted to make movies with 3DGS and if you are making games, I just don't see a good implementation of this.

So as you can see, most of what you want is already in the works!