So I wanted to start this discussion to see how ca could implement nagash (and other missing undead like neferrata into warhammer 3)
Here is my opinion (which will probably be very unpopular):
introduce nagash as a new undead faction using the ossiarch bonereapers and night haunts from age of sigmar (i know lorewise i doesn't make sense to have the ossiarch bonereapers as they took centuries to create but we have repanse so lore clearly isn't the priority, also the end times did introduce the prototype of ossiarch bonereapers with the morghasts)
Anyway I think nagash obviously needs to be in a new undead faction as he doesn't really fit into any of the existing factions (the closest we have are vampires because of their raise dead fitting of THE necromancer). The reason I chose the ossiarch bonereapers is because it's the only unique undead faction I can see which offers both good new units and several mortarchs to make up for the fact that 2 out of 3 mortarchs have already been used.
The 4 LL would be Nagash, Nefferata (with vampire/tomb king themed units on top of the base roster), Katakros and Lady olynder (with nighthaunt units).
What do you guys think?