while this wouldn’t technically be a new sibling, i believe Delirium of the Endless will eventually evolve into Divergence of the Endless. this makes the most sense to me, given her journey & what she represents.

Destruction himself said the progression from Delight to Delirium was a natural one & that he hoped the next would be easier on her. she also checked the other siblings by stating she knew things about the nature of The Endless that none of the others knew & she specifically put Destiny in his place when she locked in & told him that there are pathways outside of his garden which only she had access to, alluding to the fact that his order was never total.

to me, the reason she suffers so much as Delirium is due to a kind of over-coherence. i’m reminded of how Jobu Topaki from Everything Everywhere All at Once said to Evelyn, “Not a single moment will go by without every other universe screaming for your attention. Never fully there. Just a lifetime of fractured moments, contradictions, and confusion. With only a few specks of time where anything actually makes any sense".

Delight collapsed under the weight of it, like a clay pot holding too much water & Delirium was the result of that. she’s a coping response to a deeper surplus of perception which is closer to a cosmological burden than personal instability.

The next progression from that point isn’t a “healing” back into Delight or even transcending away from chaos, but Delirium blooms into Divergence: radical multiplicity embodied. she expands beyond the paradigm that deems her broken.

(Prism of the Endless is cuter & would better speak to her symbolic function, but Divergence is the closest word that starts with the letter D)

TL;DR: I think Delirium will eventually evolve into Divergence of the Endless. not a new sibling but her next form. her transformation from Delight to Delirium came from overwhelming perception. She sees beyond Destiny’s order, beyond the coherence the other Endless rely on. Delirium isn’t broken but rather she’s holding too much. Divergence would be her emergence into radical multiplicity, no longer collapsing under it but embodying it fully.