That avatar of Ynnead would make a great AOS Daemon prince.
Well this is just a wild speculation/question. Has it been confirmed that Slaanesh is staying around. If not i could imagine that Slannesh being "Beaten" (In some shape or form) in the 40k campaign could be the series "Big thing that happens" and would eliminate the rpoblem of having he/she in 40k but not AOS ...... Dont shoot me slaanesh lovers just popped into my brain . ...