Note: This is an older version of my prediction "Hollyleaf is Alive". This version was written before "Fading Echoes" was released.
I could be wrong, but it seems to me that most people here seem to think that Hollyleaf is dead. I disagree with this opinion, for several reasons:
First of all, it should be noted that Erin Hunter refuses to reveal any information on whether Hollyleaf is really dead. (See Erin Hunter Chat #6) This is one reason why I believe that Hollyleaf is not dead. After all, if she is, what's the big secret? She could eaisly be brought back into the series, it wouldn't be the first time. After all, look at Graystripe.
Another reason for my belief that Hollyleaf is not dead has to do with the cause of her 'death'. Think about it, of all the ways that Hollyleaf could die, why have her die in the tunnels where no one can confirm her death? I believe this was done intentionally. There are probably many more undiscovered entrances to the tunnels, leaving pleanty of potential for Hollyleaf to return at a later date.
Another interesting fact supporting my theory that Hollyleaf is still alive has to do with a major event that happened after her death. Remember the drought in The Fourth Apprentice? Well a drought means no rain, no rain means no flooding tunnels, and no flooding tunnels means that Hollyleaf could have survived for quite a while in the tunnels, even if she got lost for several days. Towards the end of the drought, Jayfeather even started looking for the opening in the tunnels that came out under the lake, believing that even this location would no longer be covered by water because of the drought. Although he never found it, this could be another event pointing to Hollyleaf being alive.
Another thing I noticed is that most of the books avoid directly saying that Hollyleaf died. Instead they usually say: "Disappeared" or "Was Lost". For example, in The Fourth Apprentice, it says that Hollyleaf "vanished behind a fall of earth". This could be another indication that Hollyleaf is still alive.
One other possible piece of evidence indicating that Hollyleaf may be still alive involves what may have been a slip on Erin Hunter's part. She stated in a video interview displayed on warriorcats.com that Omen of the Stars will feature Jayfeather, Lionblaze, and Hollyleaf as main characters. This may have been a mistake, or it may have been referring to Hollyleaf as being a main character in Starclan, but this is an interesting fact nonetheless.