We're actually in the middle of our third collision overhaul, rebuilding the foundation of the collisions system to be more robust, faster, and more accurate (and more directly linking the n-body physics with collisions). Nod to Thomas.
Once that's done, Georg is excited to begin an overhaul of the collision visual effects (glows, sparks, flashes), of which progress was frozen after we realized there would be a third overhaul of the core system.
And the past few weeks Georg has been working on crater impacts... throwing out the current system that just adds a glowing decal for a system that creates an actual 3D crater on the surface.
And we're working to make the comets and volatile trails more robust and interactive...
In short... lots of new stuff is in the works.