Here's a nice speculative thought.
We still don't have an Ynnari release. Ynnari coming out at around the same time as Slaanesh would make a lot of sense. That could definitely point towards a big coming release and make the Slaanesh/Fulgrim rumors have a bit more weight behind them.
I like that thought. I have kept an eye out on AoS concerning developments on Slaanesh's situation there, and with the buildup to their 2nd Edition in Malign portents, it was revealed that the elf factions were "fishing" souls out of Slaanesh, but Morathi was taking more than she was permitted, and slowly throwing Slaanesh's prison off balance. She also said something that implied that if the portents came to pass, Slaanesh would be freed. And as far as I know they did pass, even if the Skaven tampered with Nagash's plan, it still went off.
What I mean by this is that there are moves being made on Slaanesh in both settings, and they both have something to do with Elves and Death. I personally doubt any of these teasers have anything to do with Slaanesh (yes, even the weird one everyone says it's a Noise Marine helmet), but something's being set up, lore wise. Slaanesh is coming, whether or not that comes before or after World Eaters, that's something we'll have to see.