The Great Betrayal!

[EdwinLi]

That'd be one heck of a plot twist, assuming it succeeds. May end up being a regular solo instanced fight if everything plays out like that.
But as mentioned elsewhere, the Main Scenario teaser for Before the Fall Part 2 shows her back in the material world, so if it does succeed it'll be very, very short-lived.

Well anything can happen at this point with Minfilia's capture. If the use of Minfilia as a Host for Zodiark works then it is possible that Zodiark is hiding his presence in Minfilia or pretending to be Minfilia similar to how Lahabrea pretended to be or hid his control over Thancred's body.

If their plans failed in 2.5 Part 1 or there is another reason for Minfilia's capture and we save her then meh she still herself and nothing more.