This post was written in a darkened hotel room using an iPhone, so apologies for any grammar or spelling poo that the lack of a better spell checker provides. ;)
Over the last couple of years something I've often heard people that play tabletop warhammer and WAR say is "why did Mythic choose Tzeentch for Chaos?", normally followed by a statement about why this or that Chaos god would have been better. To find some possible answers we have to consider how you make something playable, enjoyable and attractive, yet stay within the boundaries of the well established (and restrictive) warhammer lore. I imagine that for a computer game designer this must be quite a challenge and at times a great big dollop of frustration. Yet in this case I think Mythic would have quickly realised that they had only 2 viable choices; Chaos Undivided or Tzeentch.
To see why I think this was the case we first need to look at why the other 3 options would (again, I think) have been ruled out. Now what follows in this post is not just about smothering ourselves in Warhammer geekery, but is more about how you pick a product from that geekery that will appeal to people that don't play or care about warhammer tabletop games AND at the same time not piss off the warhammer fans.
The god of rage, murder and violence. He also despises the use of magic, for that is the way of cowards and fairies. His followers are blood crazed psychotics that charge headlong into anyone and anything to claim more skulls for their equally bat shit crazy deity. Hugely popular with Warhammer fans and utterly impossible to make work in the template Mythic had for WAR. How do you make 4 classes that fit the WAR mould of Tank, melée DPS, ranged DPS and healer?
The first two are easy, but the ranged and healer are impossible. Any Khorne class has to be about charging at the enemy faster than Posh Spice runs from food and they most certainly don't heal things. So despite the initial atmosphere and blood crazed amusement, unfortunately it would get dull very fast. It's just too one dimensional having a race of 4 classes with exactly the same playstyle and the lore in this area is not flexible enough for it to be different.
The god of decay, disease and despair. This could work really well if it wasn't for the visual style of players being rotting, disease riddled and maggot infested walking corpses. Everything would be green or brown and look utterly foul, but also rather drab and depressing, which is just not going to appeal to enough people and quite possibly make too many feel a bit queasy. Plus look at the popular imagery for armour in other games, film and TV. Be it curvy or angular, it's always clean lines and well defined shapes. That just isn't an option with Nurgle.
The god of decadence, desire and unrestricted pleasure. Or in other words the god of shagging EVERYTHING... animal, mineral, vegetable, no form of depravity is too low for them. If it exists in material form then it's a viable option for a good rogering. And therein lies the problem, the all too common combination of ignorance and homophobia that is rampant in gaming, or perhaps just in people in a general sense. In my many years of being interested in Warhammer games, far too often I've heard people say something along the lines of "isn't Slaanesh a bit gay?". Which actually no it isn't and it's a little concerning if you really think that (stop reading the Daily Mail, take off the tinfoil hat and try some rational thinking please), but regardless it's far from uncommon to find people saying this. That's something that would get in the way of making this an appealing playable race for the masses. Plus artwork wise it would all be fairly risqué, gothic, murder porn which may not be so great for the advertising guys to work with. Fun times in the design studio mind you...
So for those three options we've got one that would be popular, but boring to play and then the other two wouldn't be that popular outside of existing warhammer fans and would alienate or offend people. Which leaves us with Tzeentch or Undivided.
This really could have gone either way, both are perfectly viable and it must have been a tough call. Undivided is the worship of all 4 chaos gods equally and would have given Mythic the widest range of options, with mastery trees based around different gods and their associated playstyles. But Tzeentch while more limited than the immense freedom of Undivided, it does have quite a few advantages.
Firstly it has a distinctive visual style, which would (and has proved to) be popular. It's one that gives your art team a lot of freedom, but still maintains a clear and instantly recognisable theme. It was also never going to offend anyone.
It doesn't require any restrictions to what type of gameplay you make for it's characters, it easily fit the MMO standard template.
Lore wise it is Tzeentch and his worshippers that tend to organise all the alliances, I'll fated as they maybe. These buggers are the catalyst and manipulators of all things chaos. It just makes sense for them to be running the destruction show.
Also by not going undivided and instead sticking to a one god theme, it leaves the others as options for future playables, PvE and any other options they can think up.
Visual mass appeal, no lore breaking required, wide ranging options for playstyles, less future limitations and of course, last but not least, it meant they got to make Magus on discs. Which obviously just had to be done.
That's what I think anyway.