Who was Kaguya up against that made her build an army?
I suspect that the 'real' reason is lost to redaction.
In the story of The Bamboo Cutter, from which the tale of Kaguya-Hime originates, Kaguya's people return to take her home after her stay on earth.
In some versions, she is being sheltered from a galactic war and the gold found in the bamboo cut by the old man was a form of reparations. In other versions, the gold is a sort of paranormal gift from Kaguya to show her gratitude.
I greatly suspect that the "original plot" would have Kaguya building an army to repel attempts to remove her from the planet. See - the 'original story' would have Kaguya consume the fruit and then leave with her people. Her role is to consume the fruit and properly disperse or harness its power. This keeps the power from falling into the hands of warring beings upon the planet but also keeps the tree spirit appeased.
But, Kaguya fell in love with the planet and didn't want to leave. She endeavored to help people.
There's an interesting part of the story. When her people return, they place a hagoromo over her shoulders, and her compassion for the world disappears.
It was Hagoromo, the Sage of Six Paths, who attempted to get his mother to leave the planet by sealing away her compassion for the world.
Which had the side-effect of turning her into a dictator and sociopath. It didn't rid her of her goals of ending human suffering or of staying on the planet - it just removed any consideration for what people felt about her selected method.
Within this story, her people would have begun to mount an invasion force to contain the situation. Hence, she was building an army to repel the invasion force sent to contain her.
This would also be where the Hyuuga would come from. The Hyuuga in this story were not descendants of a last-minute insertion of "Hamura" - but were the descendants of warriors who survived the invasion and remained to safeguard the planet against Kaguya. This would also be one of the reasons why the Hyuuga cannot (at least, per anything I've seen to date) use ninjutsu. We do not even see them using a substitution.
The Uzumaki, therefor, were the shamans of the tree, and the father of Hagoromo.
Thus, when Neji said that Naruto and Hinata 'share the same destiny' - this would be quite fitting, as it was an Uzumaki and a Hyuuga who started the whole mess, and an Uzumaki and a Hyuuga who would put an end to it.
But fitting all of that into that 700 chapter limit... let's bring back dead hokages (you know, because this is about the present generation) and have hundreds of panels of flashbacks reminding us of what is well established history by now since it's part of a flashback every five chapters.
Of course, it could also be that I just don't like to accept silly endings and therefor endeavor to construct my own from the first half of the story and then some.