Alright. I've sat with the ynnari rules for a while to get used to them, and I still feel like they aren't in a great place. Getting nerfed is all well and good; they needed to be toned down. But the frustrating thing is that they just aren't very
fluffy. They've lost that eldar family reunion razamatazz. Now, simply giving a ynnari detachment the ability to mix and match would be too
much, but I think the following simple changes would make it much easier to play a fluffy ynnari army without feeling that you're jumping through hoops to play a less powerful/fluffy ynnari army than you would by simply not using ynnari rules at all.
A.) Any asuryani, drukhari, harlequin, or corsair detachment in your army may be declared a ynnari detachment during list creation provided your army does not contain any coven or mandrake units. All units within such detachments gain the "reborn _____" keyword per the current White Dwarf rules. This swaps out your stratagems, psychic powers, etc. per the White Dwarf rules. Corsairs gain the "reborn space pirates" keyword. Or something less silly sounding.
B.) Unique characters can be included in ynnari detachments but do not gain the "reborn ______" keywords. Eldrad, the Avatar of Khaine, and Prince Yriel are exceptions to this because fluff reasons. Ynnari players are required to refer to their Avatars of Khaine as Meliniel or the War Shard because fluff reasons.
And that's it. Ynnari could stand to have some stratagems and psychic powers looked at, some generic
HQs added, and certain special characters added, but those would be part of a more thorough overhaul. The rules above don't do much to power up the ynnari, but they do let you form a coherent ynnari army featuring any flavor of eldar without having to spend 600+ points on named characters. Plus, Eldrad and Lelith can hang out.