My idea is simple put somes legendaries lords in several races like Sigvald or the four new champions of chaos. Sigvald would appear in warriors of chaos and "Slaanesh race". If you select Sigvald in Slaanesh race, Azazel appears too and you can confederate him.
I think that it's a good idea to satisfy everybody and it's a good introduction for Nagash who has all his mortachs ( his generals ) in several races.
I think about :
_ Villitch for Tzeentch
_ Festus for Nurgle
_ Valkia and Taurox for Khorne.
_ Sigvald, Azazel and Morathi for Slaanesh
_ Arkhan, Luthor Harkon, Noctilus, Mannfred, Helman Ghorst, Heinrich Kremmler, Neferata ( when she will come) for legion of Nagash ( when she will come too lol )
Vlad doesn't like Nagash but why not.