Could Iyanden been done as a Special Character unlock of Wraith units as Troops? Well, fuck me, yes it could
. I hope they don't release supplements for other stuff because I don't want GW to charge people shitloads for playing an army they like. I'm on the side of the Eldar players here, I sure as fuck don't want you to pay $40 or whatever it costs to use your yellow Wraithguard, because y'know, I like fighting varied armies. If everyone who wants to run a Wraith army has to pay $40 for the privelige, less Iyanden armies will be seen, which will suck for the game and suck the variety out of it. We'll just see more of the same Heldrakes and Necrons.
Sure it could have been done with an SC to unlock them as troops, but the basic codex has everything you need to do that now: the Spiritseer. Take 1 and Wraithguard and Wraithblades are troops.
This book just takes the idea further with the ability to have a Wraithlord or Wraithknight warlord, a Runes of Battle Primaris that gives Wraithguard/Blade units in 12" Battlefocus and Furious Charge (S8, AP2 axes on the charge) and up to 5 Spiritseers in a single HQ slot.
This book doesn't replace what the main codex does, but it lets you take the idea even further and I like that.