Voted for Night Lords and Iron Warriors but I REALLY want the World Eaters ...since I doubt they will let us create custom War Band color schemes .
Won't happen. I have seen a few interesting color setup in Space Marine multipayer. I guess they want to avoid that. The highest level of customization will be, that I am able to choose, that in my Iron Warrior themed CSM I am able to select the position in my legs of the yellow-black pattern.
Yeah they've stated multiple times that there will be no color customization. Which is great because Space Marine's MP was silly with every single space marine being colored differently. The customization will most likely be aesthetic changes and cosmetic items. Different helmets, stuff like that.
Returning to the original: what if the fourth "legion" would be Chaos Daemons? I guess that would be pretty awesome. It has endless possibilities.
Daemons? Really doubtful there, however it's probably a possibility in the future if they ever think about adding "Allies".
Regarding daemons: I really wonder if it will be possible to make a dark pact and become Possessed in this game.
Night lords. Even just in the game at first as an introduction. Just imagine entering an open battlefield & witnessing a Lighting Raid... You look up to see drop pods plummeting towards the planet at top speeds. But before they manage to land the hatches of each blast open. A legion worth of Night Lord Raptors launch from there yawning edifices, there winged jetpacks screaming almost as loudly as the Death Masks upon there faces as they scatter into the wind. The Drop Pods hit first, blasting the battlefield in dirt and debris, too fast for anyone to have survived inside and causing a dust cloud of cover. Then the blasted hatches rain down like shrapnel rain as sotto forms of cover for the battles ultimate beginning. Then, with a third & final impact, the Night Lords land amongst those that do battle, there shark-toothed chainswords and axes revving into the smolder and dust cloud that is quickly dissipating. And only then do they attack... The crescendo would be that while you survive the onslaught of the Lighting Raid the Night Lords would be constantly landing & trying to erect a Chaos Beacon. If successful a warp portal opens up, releasing Fury Daemons & eventually Night Lord Chaos Marines & Havoc units for support. Towards the end of the reinforcements inevitably a Chaos Lord would pop up with his elite unit of Chosen Terminators & the final battle will commence to cleanse the battlefield of there attack. And that my friends is how you introduce the Night Lords.
That's great and all "friend ". But even with the designs and the tactics that are what made popular : the Night Lords hate chaos and loyalists , think the chaos religion as a whole is a joke , they never use daemons to help themselves , they rarely ally with the other chaos forces (As in almost never) , they have one of the purest geneseed appart for a very pale skin and the ability to see in the dark so you can never get mutations or daemonic possession and they don't even live in the stinking warp .