At its current shape, Daemons of Chaos are "Daniel's faction" unfortunately - and some Persons here consider them as a "forgotten Army book". When CA introduced Be'lakor as WoC LL, they complained that they will need to make a big re-design Daemons of Chaos to add Be'lakor there.
Is that means that they will stuck with 1 not very unique LL? I hope not, and here is my idea how to re-introduce Daemons of Chaos as an army into WH3.
Here is a thing: monogods already owns the greatest of big daemons and I guess that heralds as Masque or Skulltaker will be added to monogods as a FLC. However, there are also some "second in rank" greater daemons present in Warhammer universe. With Festus, CA explained that they want to make each lord unique in some way and avoid duplicates with them. Because of that CA will not add Ka'bandha to Khorne faction, as we already can play as Skarbrand. But this character can be used for Khorne legendary character as a leader of "pure" daemon faction and then transfer this nice Forge World model into game: https://www.forgeworld.co.uk/en-ES/ka-bandha-daemon-general-of-signus-2022
Same for Tzeentch: we have Kairos already, but tzeentchian daemon LL for DoC could be Amon 'Chakai (btw, Red_Dox - many thanks for your lore articles, you are doing great work with them here
I have no idea who could lead Slaneesh faction for DoC... I think that "second in rank" for Nurgle could be Rotigus - but I don't know if this character is existing in WH 40k only...
What about roster - nothing new here, there is no missing daemon units nor heroes in WH3: there are only characters which are not top 1 and they came from lore and/or supplements. And this is a very challenging situation for CA if they want to squish more money from DoC and provide us factions worth to purchase and play. Mechanics will need to shine and be very different from things which we know, because they cannot duplicate Ku'gath plagues for DoC Nurgle character as they've done with Festus.
I am very content that Warriors of Chaos - my favourite faction - finally received some love and attention from CA - and I think that in general, Team has made quite good work with CoC DLC. But pure daemon only armies - not mono-gods with mix of daemons & warriors - deserve some attention too. I - and I am sure that some other people too - would like to play campaign with DoC army also.