Do you, or anyone in this community have access to AREPO? It seems that code is still fairly non-public. I am sure some of us could get (possibly me included) for some specific science case, if you ask Volker or Lars. I in fact don't quite know what they store. They could store particle-like data, or grid-like data or both. If they store particle-like data, I'm sure they output all necessary hydro quantities to estimate the density everywhere in space, also where there is no particle (which in a Voronoi tesellation of space would just be the density of the nearest voronoi cell center, i.e. particle, right?). So that always needs the knowledge of zero neighbors for particles, or 1 for points in space other than particle positions.
I have no direct access to AREPO data but I recently spoke with someone who is analyzing a cosmological AREPO dataset. It turns out the AREPO has an identical data format to Gadget. In fact, at least in the Hernquist group, they are using the exact same scripts to analyze Gadget and AREPO runs. The Voronoi grids are never saved to disk, so if one wants to infer what the Voronoi grid was during post-processing, it would require a re-tesselation of the domain.
Sunrise actually does this when it is run on AREPO data. This is accomplished by linking against the AREPO tessellation routines when one compiles sunrise.
The upside of is that we can legitimately say that we support AREPO datasets as soon as we finish support for Gadget. Doing AREPO support properly by self-consistently accounting for the Voronoi mesh will be more difficult. We probably want to hold off on making a big deal about this until we can find someone who uses AREPO data and is interested in becoming involved in yt development.