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if both stormcast and nighthaunt are getting their respective new waves, are we looking at night goblins and drakoath in the near future as well? ... and also, can we expect marauders to transition from slaves of darkness to this new drakoath army (given the similarity in design to the chieftain & warqueen), or would be preposterous to believe such a move could happen this late in aos? ... really think marauders look like something drakoath could have as their core unit

This is definitely my theory as well. Goblins even seem to be getting a couple of rumour engines with that small, janky looking blade and squig mouth.
Wouldn’t know for sure whether they’re doing a Daughters of Khaine with either of them and adding new units to preexisting units or doing full releases with them though. But GW even mentioned (on Facebook I believe) that 2 other factions were getting new models this year so I’d say it’s definitely possible

It's a pretty good assumption based on the other two heralds getting an army. Couple that with some of the Rumor Engine pics, and Moonclan is probably coming sooner rather than later. Based on rumors and such, I'd say we'll get Moonclan later this year, probably following the 40k Ork release. Darkoath will be sometime in 2019 if I was betting, probably Q2.

that said ... what about the prospect of marauders shifting from Slaves of Darkness to this new subfaction: darkoath???

As far as has been said, isn’t Slaanesh supposed to hit in October, before some of these?

Based on how Tzeentch and Nurgle were released, Slaanesh will get a 40k release in October/November and an AoS release in January/February.