That rumour also indicates new world eaters. I somewhat doubt that, given that the Chaos Marine codex clearly had their legion rules covered. The Chaos codex had large "Thousand Son" and "Death guard" gaps which were clearly going to be filled. I don't think GW would be releasing a Khornate faction so soon (we had Khorne Daemonkin only a few years back) and including them in the Chaos Dex.

I also agree that the next is likely to be loyalist, and given the prominence of the Fallen (back in the Chaos Dex, new Cypher release) i would suspect the lion as being the most sensible one to next appear. Like Roboute, he would be "just waking up" and thus his character doesn't have deep questions or issues as to what he has been up to for 10,000 years. Also, I feel that it would take someone like the Lion to bring the Primaris into the Dark Angel chapter convincingly.