Looking back on it I have a mixture of fond nostalgia and head-scratching frustration for the old “3.5” version of Codex: Chaos Space Marines. I loved how that book worked even if how it worked was ridiculously twisted at times (seriously, some of the rules were hidden in the fiction sections). I loved how you got all sorts of perks for having an army exclusively dedicated to one Chaos God or wholly representing a single Legion. I loved that there were whole pages of god-specific wargear and vehicle upgrades.
So you can imagine that I'm reeeeeealy looking forward to Codex: Khorne Daemonkin because this is how I see Chaos armies: not as Chaos armies but as Khorne armies or Tzeentch armies and so on. I know I'm not alone among my generation of hobbyists that I could never get used to the “pantheonic” approach GW piloted with the first Chaos Daemon/Daemons Of Chaos release.
I've never really got used to how Daemon armies work. What I want is an army dedicated to a single god but the army is built to work best with the pantheonic approach. Plus, there just aren't enough units for a single god to stop me getting bored.
All this said it isn't the Khorne Daemonkin that I'm excited about. I'm hoping there are others in the works, specifically for Nurgle and Tzeentch Daemonkin warbands. I love the Death Guard and Thousand Sons background and models (I have a bunch of Forge World Death Guard in a draw ever ready for deployment) and I'd love to do warbands with those themes.
Plus, Renegades & Heretics should be Battle Brothers with the Daemonkin so there's even more variety and potential for fun conversions.