Hadn't been following Neferata's story, but it kinda sounds like they're lining up the Mortarchs for new (possibly separate?) kits down the line.
Especially for Arkhan - his dramatic exit in BR seems like an opportunity for him to make a big return. Maybe he and the other Mortarchs (from their shared kit) get models with a bigger base size, to represent them gaining more prominence after Nagash's defeat?