As far as if Chaos Space Marines or Necrons or Dark Eldar would be better for the Starter Set, there is one other 'trend' we're seeing that can help with that. For the last two starter sets, GW has used the starter to fill in holes in their line. AOBR had Nobz and Deffkoptaz and IOB has Swordmasters, Sea guard, the Grifffon, Reavers, Rat Ogres, and the Skaven Weapon teams. Though the starter is mainly marketed towards new players or kids, this decisions gives veteran players a reason to pick it up as well.
So what does this mean for the 6th Edition starter? Well, we're all basically agreed that Dark Angels are one side in the box. Though there can be some more specific kits to make them more like Dark Angels, I'm guessing they'll stay more general to keep people's options open. If that is the case, there is not really anything to fill in for the Dark Angels. Not saying that it couldn't happen, but Space Marines are generally covered as far as kits go. For other forces, you could go with the latest updated book, Necrons, but when you look at the amount of units that need to be updated or created, there really isn't anything to put in. Likewise with the Dark Eldar. Both of these limit the potential of sales. Yes, the set may be mainly for kids, so you may say new kits aren't needed as these kids don't care about what's in it. On the other hand, if you include new kits, veterans may be more interested in the set, expanding the potential buyer group.
Which brings us to the much-talked of Chaos force in the set. If they were to include it, there is more potential to include new kits. Chaos has quite a few old metal kits such as the Dreadnought that Veteran players have been asking for. And if rumors are to be believed, they are also getting things like cultists that Chaos didn't have already. Chaos players may also have a use of the Dark Angel forces if they're willing to convert a little. GW is likely to think that making sure people including veterans want to buy the set, the better for them.
So yes, GW may not follow this idea, and may go with another force. I'm not saying that Chaos are in for sure based on this assumption. But it is something that people seem to be ignoring in favor of things like "xenos=better bad guys," "starter force=new army" and "Chaos=too similar to Space Marines."