Archer, how interesting would it be if Sonia did have a change of heart?
She helped encourage the humans to fight with the apostles, sure. And she kind of is weirdly comfortable being in the presence of demons. But during those moments she's thinking (I'm assuming a bit here) that this whole war Griffith is waging is for the good of humanity.

I don't think we exactly know what the God Hands end game is though we can assume from what we know it doesn't involve the human race living happily ever and all the demons finding day jobs.

We're entering "throwing stuff against a wall" territory but how about this:
What if one day Sonia has a vision of what the God hand (or specifically Femto's) end game is? What if she sees her poor little owl buddy Schierke impaled on a pike, mountains of corpses surrounding her in this vision? Do you think her school girl infatuation with Griffith will hold sway over someone she can actually relate too like Schierke?
How interesting would it be if Sonia became Guts ace in the hole? What better advantage to have against a being with god-like powers than knowing his next move? You could go all sorts of directions with that kind of theme.
Now I don't think it will go down exactly as I described because that seems a little too simple for me, but there will definitely be some shenanigans involving Sonia & Guts group and I am betting it will work out to the latters advantage.