Karanak is a 3 headed flesh hound.
Scylla Anfingrim is a massive Khorne worshipping Chaos Spawn.
Both have unique models created by Games Workshop.
Both would be candidates for legendary hero status, but it is unlikely that CA will create multiple LHs for Khorne. After all, several factions don't have any LHs at all.
What about creating either or both as a Single Entity Model (SEM) Regiment of Renown (RoR) with their special model and maybe some special rules?
Although a RoR SEM is not as nifty as a legendary hero, it would still be a way for these special models to get into the game.