Collision detection, relativity, full interstellar travel, and more detailed terrain and sky are the only big things I feel Orbiter's missing at the moment. Smooth interstellar travel plus relativity would really be almost a whole new sim.
In the interim, the only improvements I'd be really excited about would be cosmetic:
- Lens flare
- Bloom, or some kind of glow around objects on their sunlit side... It could be subtle around Earth's orbit, grow to a frightful glare around Mercury's orbit, and fade to nothing past Mars or so. It'd be a great way to add some "character" to areas of space, making hot and cold areas a little more visceral.
- A way to get rid of those "seams" in clouds you see in LEO. Every time I see one of those roll by under my ship I'm taken out of the experience for a moment (I have no idea if closing them up is possible, though)
I'm down with what some people already said about seeing little procedurally-generated details as you near the surface of a planet, like boulders of various kinds, or on Earth some little trees or buildings. Really, I'd be satisfied with trees just being a single sprite always facing the camera, like smoke trails are now. The same thing might even be doable with buildings, but you'd have to be very clever about drawing and placing them, like how an impressionist painter can suggest a small, far-off house with just a few brush strokes. Ultimately I'd rather have just a whisper or a suggestion of ground detail than a full, system-resource-and-developer-time-hogging full recreation of terrain.
As long as we're making wishes, I also would love to one day see a UMMU walk cycle.
Really, like someone else already said, we're in it for the detail and realism, so more of the same, please. But to me, immersive graphics and other elements are part of the realism, so...