May I ask what's the holdup? Comets are one of the most important and prominent phenomena in space. One should think their visual models would have been in the game a long time ago.
Honestly, while I have no inside knowledge doing comets right would be challenging. In the first place, their typical highly eccentric orbits likely fall a bit outside any "sanity limits" that might be imposed generating a star system. That's tweakable but given the procedural methods used to generate star systems could easily have undesirable effects down the road. That makes the effort of doing so non-trivial for FD. Since they currently exist as point masses, let's assume that FD have cracked this one as that would likely be a prerequisite for even including the mass. Visually, a comet core would likely be too small to show up on a scanner unless you were right in its frame of reference in normal space, much the same as individual rocks show up on your scanner in ring systems and asteroid fields. Since the game can do that we know FD already have
most of the code they need for this part but again, it would likely require modification. Since the game can render asteroids probably an individual rock is not beyond the bounds of possibility either. Now we get to the stuff that we DONT see similar examples of in the game already. Rendering individual off-gassing jets and the coma and tail. While the visual effects for approaching and flying through these would likely be somewhat similar to those experienced while flying within a nebula, nebulae in the game are largely static whereas the volume and intensity of those visual effects for a comet would be highly dynamic, varying by proximity of the comets nucleus to any stellar bodies in the system and by the vector(s) from any nearby star(s) - They'd effectively have to model the direction and intensity of the solar wind at the comets location in real time, along with the amount of surface heating the comet was experiencing and a host of other factors.
All in all, I'm happy that they are intent on doing it as correctly as possible rather than throwing in a visual placeholder. The point masses are placeholder enough until they can do the visuals well.