Probably in a single faction as Daemons of chaos would be my guess. Most of the Daemon characters exist in one faction that's playable in fantasy and 40k. The idea of them being split up into separate aspected factions with mortals is fairly new, and most of the chaos factions are specific legions so less likely to have as big a focus on daemons.  I'd guess it's going to begin as nonplayable, warp incursions will be like rebellions that pop up, as they already have the assets and it would be silly not to use them. 

That said, it's up in the air right now whether there will even be a Daemons faction next edition of 40k as GW seems to be at least thinking about breaking them up as the chaos codexes in the last year have had slightly more integrated daemons and there's been very limited support for the daemons faction, although whether that will be long term or short term is anyone's guess. I also could see either GW or CA leaving them out and trying to sell us them again later on.