It’s very easy to add a custom reader/writer plugin to Slicer. It’s just a hidden scripted module that registers a subclass something like this. Adding NPY/NPZ in a similar way would be great.
But we also want to support at least the direction and spacing info as metadata. We’ve been discussing using xarray and/or zarr for these probably in ngff. I know @thewtex has been working on that and I think it is a good direction for Slicer but we haven’t gotten too serious yet. Here’s and example zarr reader for ngff.