All the heroes, past and present, coming together to convey to the Shinju that they choose to fight as humans, convincing Shinju to sacrifice itself to give Yuuna the power to destroy the invading sky god(s?). It was a fitting way to end off a series where the themes have always revolved around sacrifice and what you are willing to sacrifice of yourself for, having it turned around where Shinju offers itself instead. Granted, given how dire the situation was, it's about the only option that was left at that point...
Taisha is gone, good riddance; as much as they were trying to preserve humanity, their half-truths and hero systems they came up with were rather twisted... why couldn't they have come up with this very last iteration earlier on, which actually is a reasonable design, compared to the last?
So humans are on their own now. Question is, are the other sky gods still around, or has that problem been permanently dealt with? (Some speculation on the symbolism behind the mirror breaking is that they've been sealed off from humanity, but I'm unfamiliar with the mythology behind it to say anything myself). That might be the end of the storyline here, but there's always prequel material that can be done as well, I suppose.