I'm starting to think Delryn might be set up to replace Sylvanas not as the Leader of the Forsaken but as a Sylvanas-esque character. That could be apart of Sylvanas big plan to have a Sylvanas-like person in the world when she's gone.
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Teldrassil= Quel'thalas. Delryn resurrected= Sylvanas resurrected. Kinda.
They mirror each other extremely well and Delryn being apart of the Forsaken is interesting. I don't think they need to make her an allied race leader but a commander of the Forsaken is well enough.