The trailer itself brings some interesting ideas into play, however. In the trailer, the pools of "green energy" are stated to be "ripples of time", which I`m assuming is a new concept of time travel for Celebi which they`ve developed, perhaps based off of the Mystery Dungeon games. Anyway, at te end of the trailer, it shows Celebi emerging from one in a present-day city saying "At that moment/But then, Celebi has seen the future!", implying that it may be trying to alter the course of history.
Now, my personal theory is that Ho-Oh and Lugia will still be involved. Given that the Legendary Beasts seem to be taking action in the future (in the trailer), it could be possible Ho-Oh is as well, and they are gathering to assist it. The teaser trailer showed Lugia and Ho-Oh battling, and while I doubt they actually would battle just like that, it gives the possibility of the same thing happening for Lugia and it`s trio. Either way, I`m still going to assume Ho-Oh and Lugia will be in the movie. Hopefully we`ll know y next month`s Coro Coro. And I`m personally hoping for a new Pokémon as well