This is based upon the information from Bleach season 1 and all relevant hollow information I could get my hands on.
This is specifically targeted at the Ulquiorra fans (including my girlfriend) so if you wish to post this to others whom are sadden by the current events in the manga.

The Fowler Arrancar Theory
My theory is that as stated in the first episodes of Bleach season 1 all hollows DO NOT die, they simply pass on to the soul society as a purified soul.This has never been a statement revoked when being discussed about Menos Grande or Arrancar.
Now as Bleach hardcore fans will know the Menos Grande are a group of Hollows with one staying as the dominant entity, then when a Menos Grande goes through Shinigamification (a real term from Bleach) leading to that one entity becoming an Arrancar with the spiritual pressure of the rest combined with their own.
Now by putting the first and second statement together this would mean that when an Arrancar is 'killed' it will become purified and become a resident of the soul society as all hollows do unless being sinful before dying.

Now back to the Ulquiorra fans this would mean Ulquiorra will not die but become a human spirit and live in the Rukongai and could eventually be a soul reaper! This would show Ulquiorra as his human self.

I hope this can give even a small glimmar of hope to all Ulquiorra and any other arrancar fan.