I'd like to think Ulquiorra is not gone.
Firstly, I think it's significant that Kubo titled the chapter "The Ash" rather than "The Dust." Really, if he was going for "ashes to ashes, dust to dust" he could have used either, or both. But he didn't. He kept it simply "The Ash."
What comes from ash? The phoenix.
Some info on the phoenix from Wikipedia.
The bird was also said to regenerate when hurt or wounded by a foe, thus being almost immortal and invincible — it is also said that it can heal a person with a tear from its eyes and make them temporarily immune to death.Does this NOT fit Ulquiorra?? He regenerates. He has stripes under his eyes that could be construed as tears.
Secondly, what was it that Hacchi said to Orihime?
Here he says, "From just looking at your reiatsu, you should have powers very close to mine...but if that was the case, you would be able to return this to its original form...
even without a shattered fragment." (Bold emphasis is mine).
And
here, he mentions that "What's important is not 'how it should be' but rather '
how do you want it to be?'"
Additionally he mentioned that she
hadn't mastered her full abilities yet.
Orihime is the only human nakama who hasn't really received a power-up yet. Bringing Ulquiorra back from ashes, simply because she wants it that way, could be her finally realizing her full abilities.