These would be for the Warriors of Chaos update. I'd see new marked characters, Valkia the Bloody, Festus the Leechlord, Egrimm van Horstmann, and either Dechala or a revamp Sigvald. As well as new marked units: Rotbringers of Nurgle, Hellstriders of Slaanesh, Skullcrushers, Wrathmongers, Skullreapers of Khorne. However, there was never any special marked unit for Tzeentch in Warhammer Fantasy and the AoS Arcanites don't fit with the Warriors of Chaos vibe.
The other issue is that there are alot of good units that come from the End Times. The aforementioned Rotbringers, Wrathmongers and Skullreapers are End Times as well as some unique Nurgle Chaos characters. The Glottkin, characters riding Maggoths. Khorne character Skarr Bloodwrath.
Another point is the plethora of Chaos characters that will probably go unused but would be great to add. Galrauch as the Chaos Dragon, Aekold Helbrass who could be a Lord with access to the Lore of Life, Harald Hammerstorm (the guy on the cover of Warhammer Fantasy First Edition).
And don't forget non-Chaos characters that should be added. Valten for the Empire, Nagash, Ushoran, Neferata, Aborash for Vampire Counts.
Feels like there should be a Total War Warhammer 4 just to fill in the gaps.
Edit: Forgot that Tzeentch could have Vilitich the Curseling as a Lord. So many characters!