After all the times we've seen Ho-Oh on TV, I highly doubt that Japan would go back in incorporate him in having some big story for theaters. I mean before he went off to go to Hoenn, ASh saw Ho-Oh. In the original Movie 8 trailer, we had Ho-Oh, but that was just a Mew using 'Transform', and then there was the Ho-Oh story from Johto League Champions withe the Ecrutek City episodes.
We had our chances of seeing Ho-Oh front and center on the big screen or even on the small screen since it has been in the introductions of a few of the films so having him in a film to end Ash's story might've been a good idea but that opportunity was thrown out the window just like the story of Ash's father and a possible origin story of the GS Ball.