I wouldn't mind having 3D models overlayed onto the scene such as a transparent sphere representing the Oort cloud and a circle showing how far radio signals have gone since they were first broadcast. Currently in SE it can be difficult to tell how fast you're going and how massive a scene is due to lack of objects to compare scales.
My idea is to be able to spawn in 3D models, including stuff like Sol's heliosphere, the M87 black hole, IC 1101, etc. near something like the Solar System and be able to use a simple GUI to move the objects around, set transparency and rotate. It'd be awesome to be able to place the m87 black hole on top of the solar system and see how massive it is, then start moving your camera through interstellar space to get a feel for how far away objects are.
I don't expect this stuff to happen in 0.990 but it'd be amazing to see this happen at some point.