I will start to say that I have no hope this will come true but I thought I’d raise the issue anyway.
I think that Slaanesh can be done in a Warhammer game in tasteful way by simply applying a lite creativity to the concept of “excess” and “desire”.
Below are a few takes on ways in which followers of Slaanesh could be added to the game as new factions.
Circle of the Chosen Few Cult (Selfishness and Sloth) - A faction based on the concept of their various enemies draining strength from and exploiting their social lessers in the cult hierarchy.
Cult of the Glorious Master (Cult of Personality) - A faction based around imitating the cult magos and crazy cult devotion to an uncaring master.
Dance of Doom Cult (Pride) - Cult based around the concepts of dancing with fast and agile enemies coming in fast and hard, perhaps some with dodge or an attack patterns for elites and specials which are about acrobatic stunts as opposed to just running up and cut at the heroes.
The Bejewelled Tribe (Greed) - A Norscan tribe based on greed with them decked out with jewellry and much gems and previous metals and stuff. Less armour and HP but more damage output and attack speed. The idea being that as greed-driven people they are more aggressive and coming in hard and fast with freshly looted weapons.
The Gilded Claws Tribe (Vanity and cruelty) - Another Norscan tribe focused on the use of pain inflicting weapons that does less outright damage but has more of a bleeding aspect to their attacks or cause stagger on the heroes. This may got a bit on the Dark Elf direction but not overly so.
The Lindwyrm Tribe (Ambition and serpents) - A tribe led by a crazy half-serpent mutant hybrid champion who is coming south in a blatant bid to make himself a Daemon Prince. Think like a Fulgrim-wannabe. I figure their special thing could be the use of poison weapons against the Ü5.
Unending Feast Cult (Gluttony) - A cult of cannibals. A speciality could be an acolyte for the magos who make the warriors enter a gluttonous frenzy and so grow stronger the more blood they shed from the û5. One can argue that the effect is something that would fit for some kind of Khornate enemy but I also feel that it could work pretty well for a Slaaneshi cannibal cult.