I came up with a theory concerning Z's relationship to Celebi. Bare with me here:
Z, like Celebi, is able to travel through time. Also, based on the codename 'Z' we can speculate that it may be the opposite Pokemon to Arceus (A to Z, Alpha to Omega). Now, perhaps Z is Arceus's opposite, but at the same time, Z is Celebi's counterpart. Where Celebi travels through time to keep peace and protect the forests, Z travels through time to cause havoc and destruction. So, in the upcoming movie, Z is chasing Celebi through time to eradicate her and have free reign to cause destruction throughout time. Where do Suicune, Raikou, and Entei fit into this? Z is the cause of the destruction of the Brass tower in Ecruteak and also is the one who caused the deathes of Suicune, Raikou, and Entei. So, the three beasts are tailing Z when they sense that there is a disturbance in the balance of nature in order to stop Z from killing Celebi.
This is a theory based on what I've gathered from this thread, my own imagination, and the thought that Z could be the first 'evil' Pokemon.
Tell me what you guys think =)