If I recall correctly in an interview it got said that PA isn't working towards something big, however there is something else hinted and that we probably only start to see what it is from PA4 or PA5 on. So who knows perhaps it really is EC.
I think the context of that "PA isn't leading towards something big" quote was about the storylines of the different books leading to one final climax in the style of Malign Portents or Gathering Storm where all narratives merge. That doesn't mean the series cannot hint towards things happening in the near future. Maybe the series won't end with "AND HERE'S FULGRIM" or any other kind of big jaw-dropper, but EC could very plausibly show up later in the year, after PA is done.
I mean, really, when it comes to Emperor's Children there isn't much to set up anyway. They're not in the situation the World Eaters are in, with their Primarch trying to escape Warp-detention. They are out there, partying with Fulgrim across the galaxy, we know this. All they need (and I don't think it's 100% a necessity but GW won't introduce a "new" faction without fanfare, much less a Daemon Primarch) is a scenario, narrative and/or war zone in which to make their grand appearance.
This whole thing with them acting weird, ignoring vulnerable worlds, chasing Eldar but letting them live, having Haemonculi on their ships, etc. seems to be building up to said narrative or scenario. They definitely are a constant across Psychic Awakening, that much is safe to say.
Edited by DeadFingers, 19 January 2020 - 04:44 PM.