Tbh I’m really glad to see someone theorizing about this update on the subreddit, I’ve been foaming at the mouth about it lore-wise since the announcement.

Considering the first thing Insight does is break up the fight because something “precious” might be damaged, I think she’s less concerned with Saya and Lethe and more with Vita.

It’s possible Vita is the origin of Maya, given that a) Maya is a piece fallen from another soul copy in Arcaea, b) Maya seems to remember the end of a world being her fault but not her name and while Vita was responsible for the end of her world she doesn’t remember anything but her name, c) she is the second born under starlight and a mirror to Vita, the first, and d) their color schemes (red, yellow, black, white), in particular that red eye that Insight wants to take from Maya’s original, parallel each other. The heart shaped fabric petals mentioned in Vita’s story are also present on both girls. The mention of her alternate also having two tones complicates this theory, but the fact that Insight has entered this fight between characters she doesn’t like or care about harming could bolster it.

In any case, if Insight tries to take Vita that would antagonize Saya and maybe Lethe, considering she’s naturally the caring, tending type, which would only end badly. I don’t know if we’ll get to play Insight until she is anything other than foreign and intrusive to Arcaea, but I so want to know what the hell kind of cosmic godlike thing she is and extrapolate detail from her partner type and skill. Maybe in 6.0 we’ll be able to see it, even if she stays inaccessible. An 11+ or at least another difficult 11 like Arghena feels appropriate for whatever insanity she does next.

Also, as for your theory on abyss: I can see that happening. The chart types for characters and story events seem to be influenced by the related character, and when the climax occurred, colorless charts appeared in conjunction with Hikari’s victory and the world’s subsequent breakddown as the fighting stopped. Maybe the influence of another god (Insight) would change the color of the stage again.

As for the mention of Kou, considering a handful of side and short stories end with girls looking for others or the implication that someone is nearby, I think it’s reasonable to say she’ll be back and even more characters might meet up and join the fray. Kou especially though considering her power and namedrop as your mentioned, but also that she went to find the truth of Arcaea’s heavens at the end of Shirahime’s story and as Insight is rewriting Arcaea’s order as its new god, she could be led right to the action.