Creative Assembly have released what appears to be the full Steam achievements list for Total War: Warhammer 2. There’s the risk of minor spoilers if you look at these, but the vast majority are mechanical rather than narrative in nature (since that’s the nature of the game).
Among the various “have 5 trade agreements” and “own 10 magic weapons” achievements, there’s one particularly interesting inclusion. Legacy of the Great Necromancer is obtained by “Construct[ing] the Black Pyramid of Nagash.” This doesn’t mean Nagash is about to show up in Total War: Warhammer 2 at launch, but it’s interesting that they’re laying some of that groundwork.
He, alongside Chaos, is the other Big Bad threat to the world; and he has a longstanding enmity with the Tomb Kings faction. The Tomb Kings (the Ancient Egyptian styled Undead, as opposed to the Hammer Horror Vampire Counts) are widely expected as an expansion to Warhammer 2 somewhere down the line.
I’m not exactly sure which of the four launch factions would want to build the Black Pyramid. It’s a location of powerful magic, so perhaps the Dark Elves or Skaven need to use it as part of their Vortex rituals.