Gonna take an educated guess on this one but I think I see how this season ends and the Witch Queen begins.
I had thought about this last week, just as an idea that popped into my head, but this week's conversation with Savathun has me pretty much convinced as to what's going to happen. At least, in loose ideas. So, if you haven't completed this week's story quest then stop now.
Good? Okay..
We know that Savathun is entrusting Mara to rid her of her worm, yes? But what did she promise to Mara in exchange? Far as we know, nothing. Until you think about what Witch Queen is about: Savathun being a goddess of Light.
My guess is that Mara is going to betray us, in some way, to earn Savathun's favor of returning Uldren to her. I think Mara knows Savathun's plan to control the Light and, in exchange for making this possible, she's promised to return Uldren from the Light. I'm guessing obviously a lie, one way or another, but that's their deal.
Savathun speaks very heavily about what siblings are willing to do for one another out of love and care. We also know that Mara has, in her head, the idea of "a side must always be taken, even if it's the wrong side." These things, plus what we're working toward, plus what's coming with Witch Queen has me very convinced that we're going to be betrayed by Mara, as will Mara by Savathun, and will ultimately lead to what happens with Savathun and the Light.
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She's doing it to fix the Dreaming city curse, IIrc?