It can become a Slicer core module, but I think it is too early. It will take some time for the module to become more generally usable and work well for a wide range of data. If it is in Slicer Stable Release then you can update it just once in every few months. If it is in an extension then you can update it every day.
Sandbox is a general-purpose collection of maturing Slicer modules, so I think it is a good place for this module. Adding a new extension and maintaining it has some overhead, so it would make sense if we have at least 1-2 more modules for DICOM batch processing - or if the module becomes very robust and capable, so we don’t want to keep it in “sandbox” anymore but we don’t want to move it to Slicer core (e.g., to keep it easier to make changes).
I would suggest to submit to the Sandbox now and decide on its final place later.