Here I was going over this list yesterday if anyone's interested (hardly comprehensive but a good read). The issue is they have to be alive and kicking at the time of the Black Crusade, not have originated in another studio's take on the IP and would have to have their Legions/Warbands at least somewhat represented.

I like the Chosen of Abaddon, said that before, and individually they are the equal of any other Chaos Lord apart from perhaps Typhus or Huron. They were serving directly under the Despoiler's command for most of the Crusade however at least 2 did undertake missions for him away from the main engagements of Cadia and there was a leadership coup within their rank.

I can imagine Huron holding the Arkhona campaign as something of an audition if you want to get into the Red Corsair's, but since the Black Legion is an actual playable chapter, that does seem a bit... naff.

If they manage to get Eliphas the Inheritor from Dawn of War (and his old voice actor), I'm converting to Chaos this instantly.

On a more serious note, I think we won't see a cult marine (i.e. Kharne, Ahriman, Typhus, Lucius, etc). leading the charge. These are powerful individuals, but not ideal to lead an army consisting of different Chaos Legions.

To be honest, I also think that we'll see a new baddy (presumably from the Black Legion) who will lead the charge.

You guys got it all wrong, Wulfrik the Wanderer crash lands his party barge Seafang on Arkhona and rallies all us CSM rabble.

Don't some Black Legion worship Abbaddon? Could he not be the god? With a Black Legionnaire as the Champion?

Now that we had the Ork Warboss, I defiantly think the Chaos Leader is going to be created by @IvanMulkeen, I foresee him being Black Legion as well.

Cypher

I wouldn't dismiss the Word Bearers as a possiblity that easily. I agree Black Legion is very likely, but out of the four Legions we know, the Bearers' more-or-less emphasis on unity under the banner of Chaos is just as strong as the call of the Black Legion. Of course this may just be me fanboying for them.

Looking at it by way of exclusion, I believe that Night Lords and Iron Warriors are unlikely, however. The Night Lords more or less seem to have their own specialization more hardwired into them than the other groups (save for the Iron Warriors, but I believe their thing to be more broad), which makes them not as ideal to lead a combined force, and the Iron Warriors are... well, let's just say I'm basing my argument around the fact they can't ally in HH on tabletop. It seems they tend to view all lives as expendable, and I find it hard to envision a charismatic IW lord.

Not that I dislike those legions, I just don't consider them fit to lead this combined semi-crusade.

I agree about the NL and IW but I don't see the WB leading this. They might be good at gathering allies just the same as the Black Legion but the BL are more diverse and tactical whereas the WB are too fanatic and single minded to lead the warbands better than BL. The BL's specialty seems to be leading troops while the WB is gathering them (and then throwing to the grinder).

Also this might not mean much but the test models we've seen are Word Bearers and Dark Angels. There's also a Dark Angels trailer, DA artwork, Rogue Trader gear shows DA, but then we hear about Mephiston. Maybe that means if DA dont get a leader (yet are the showpiece) then WB won't either seeing that they're the Chaos showpiece.

I gotta go with Valrak on this, Since now we know for a fact they will introduce their own characters into the fray, Chaos has a high chance of getting a Newby added