There is a brilliant idea being discussed for now in the Orks branch of the forum - http://forum.eternalcrusade.com/threads/orks-vs-orks.56097/ - about orks fighting orks which is very lore-wise and would only add more true warhammer entourage to this game.

So I thought about the same idea for Chaos - indeed, since when Traitor Legions fight as one, unified force? The Eye of Terror trembled for decades under mutual rivalries of chaos space marines and the fact that Abaddon united all of them under his banner doesnt mean at all that they pursue only one agenda and simply forgave their brethren for immense mutual bloodshed. Why not to add an opportunity to battle CSM from different Legions? Just imagine - even if Abaddon captures Arkhona it doesnt mean Black Legion shall have it for other Legions will inevitably claim their right for territories.

The only problem I see is that how such a feature will affect territory contol system? Will Chaos still have one type of territories controlled or they will be somehow divided among various Legions? Perhaps some system may be introduced - say, when Chaos does well, there are no mutual rivalries amongst Legions since captured territories satisfy their ambitions but if Chaos faction loses territories then some "insurgengices" occur - it's typical for chaos space marines, when they suspect their leaders to be weak or ineffective enough, simply riot against them and choose a new warlord.

Will the same principle by applied to Eldar and LSM? It's lore-wise that chaos space marines fight each other in clans, but it would be "strange" at least to find Eldar fighting Eldar, not even talking about loyalists

Yes, I meant that it would be wrong to apply such rivalries to Eldar and LSM. Perhaps indeed some duels between clans which affect just distribution of the territories within a faction among clans, not actual number of territories controlled by it. Say, one Eldar guild or a Space Marines Chapter dominates on a particular region and owns some percentage of it, gaining additional bonuses for its members. In case of Chaos I meant that a guild completely owns a region, pushing anybody else away from it and it somehow complicates everything for Chaos faction as a whole.

Make long story short, I intended to say next: there are always some political games among Eldar and even LSM, at least according to lore and numerous novels, but if these two factions can still collaborate with their, say, less influential brethren, for a common good and purpose, then to do the same for Chaos is extremely difficult, the strongest prevails and gains everything.

I think for Chaos it would be cool if each of the subfactions had a God alignment (except BL since they are unaligned IIRC) or even if the players can join specific special God aligned battles they can opt into from time to time.

Then they can war with eachother to build up a specific Gods favor for all players like reduced cost marks for their respective Gods, etc.

So this way you can krump all the other factions and the meters go up a bit for each win for all of them but if you join a God Aligned battle you can spike up a Gods favor to discount your preferred Gods gifts.

So its not all about terrain and still representative of the Gods warring and gifting their devout followers with their boons.

Even if not full on battles letting a similar meter occur for dueling in the Garrisons by players who opt in to represent a God to boost it. Honor duels for their Gods.

When do Chaos Marines fight together as a mostly united force? During Black Crusades and most of the time they leave the Eye in force to accomplish anything important. Siege of Vraks for a minor example.

The Badab War, Space Wolves vs Grey Knights following the 1st War of Armageddon, current Call of the Wulfen lore.

Those three come to mind, and I think ther's a few more.

When god addicted sub factions are in, like Emperors Children that would be a nice idea.

But only subfaction vs subfaction.
Not same sub vs same sub.

SW vs Flesh tearers etc etc.