Okay, think I can have an idea here.....
Legendary Pokemon Capture Proposal
Since this place is still fairly overgrown with nature, I propose its here that we can get Celebi to be captured. You are leaving the temple, and walking a little far when ya see some twinkles in the distant grass. Ya start walking over only to realize ya seem to cant move to that grass because it suddenly got replaced by a wall of rocks. You get confused, only for a hiker to ask, "Bud, why are you walking into a rock formation?" The player is just thoroughly confused, and thinks to ask if there is a sort of cave opening in the wall. Nearby, an Ace Trainer says, "Yah, there is opening blocked by a bald man there. The fella keeps babbling on bout....stuff."
The player walks towards where the Ace Trainer points to, and doesn't see an old man or the rock wall anymore. You walk up to the entrance, only for it to change back into grass. The field of grass you see is all shimmering, and you end up seeing Celebi poke its head out from the grass and back into it. You are having to play a game of cat and mouse in this grass field for Celebi ( which, can be silly cause as you are going to grab it, it is slipping in and out of time, so its grass one second, rock the next).
Upon capture, the Ace Trainer is freaked out cause the cave he saw was changing to grass then cave like crazy.. He runs off a bit down the route in fright (if someone wants to, they can have him as a battle. I dont feel like it right now), and at the entrance is an old man who lets you pass while saying, "......Dang kids and their Pokemon. Never know what they'll do next."
So ya, Celebi in a game of Cat and Mouse with slipping through time in the environment it lives in (rock present, frass future). Fun fun