They just threw a whole bunch of visual metaphors in the opening. Iruka's note was a paper crane, the cannon, Naruto literally shouldering other's wishes (the treebranch)...
The lanterns are all the dead people going toward Naruto and he takes on, like their Will, which is like those notes lining themselves up to make arms that wrap around him and they're so heavy he starts to penetrate the ground. He eats Iruka's note -- the paper crane -- and the burden transforms into support. Etc. etc. etc.
I don't get the animals. I don't get half of it but it's not like it's meant to be analyzed, but enjoyed. I'm sure whoever made it had something in mind with whatever they threw in there.
Sometimes they share a few words about new openings or endings on their blog.
The cannon is like "Let's show how powerful he is by throwing a visual metaphor on there because I can't think of another way of doing that and we have to get this new opening finished."