for a start there is the war in heaven and I have additionally seen necrons mentioned as the ancient enemy (in regards to the eldar) numerous times,
if it does turn out to be slaanesh I will be fairly surprised
The Necron codex mentions the War in Heaven, but also mentions most Eldar have forgotten about them (in all fairness when you accidently birth a god who will stop at nothing to EAT YOUR SOUL then piddly things like undying robot zombies become a lesser concern).
We did get a claim on that "Ancient" thing and it looks like it's NOT "Ancient Enemy" by "Ancient Doom":
Eldar models suffer -1LD to all fear tests against daemons of slaanesh but pick up hatred against&aofc;such units. Could the Fear rule actually become useful for once
Honestly I could see this being the case. -1LD test vs Daemons of Slaanesh but you get Hated Daemons of Slaanesh? Works for me!