I actually woke up from a dream about the OGB. Well, it wasn't really about the OGB, but when I was in that place between dreaming and waking, my brain plugged it into an OGB scenario. It was interesting...
I'm not going to go into all of it, but for this part, you have to assume the OGB is the protagonist. OGB as the protag wasn't the only one where everything worked, but I found it the most intriguing because of the additional dynamics.
The scenario is that you're near the end of the game, and you have to choose between siding with Flemeth or Morrigan.
Morrigan is your mother, and you've learned she's been grooming you for this moment all your life. You're the only one who can kill Flemeth for good, and ensure that the Old Gods will never meddle with the world again. This is the only way to save the world, but it will cost your own life. However, you've also learned that you have the soul of an Old God, and you might not be sure this is what you really want. You may also resent your mother for planning to sacrifice you all along.
Flemeth, aka Scary Grandma, is promising to restore you to godhood and all the power that comes with it, but only if you join her. However, she plans to wipe out humanity and restore the world to the way it was in an earlier era, ruled by dragons and magic. Considering you've been living your life as a human, you may not want this either. Even if you've decided that humanity sucks (you've been given a multitude of reasons to come to his conclusion over the course of the game), you probably have friends and loved ones who would die.
Those are the two main choices, but it might not always result in simply "protag sacrifice" or "evil bastard scenario". Once again, there might be a way to circumvent the sacrifice scenario, but someone else might have to be sacrificed as a proxy. Maybe the OGB's father, maybe the Warden (which might be the father), maybe Morrigan, maybe a companion. Maybe your LI.
If you join Flemeth, you may realize there's a catch to her offer. Maybe she expects you to be her thrall. Maybe she intends to possess you. Then maybe you have a change of heart, or you intended to betray her all along once you got the power you wanted. You now have the power to defeat Flemeth without sacrificing yourself, but you may have to leave Thedas forever.
Seeing as how the world isn't likely to actually end in DA3, I expect that Flemeth's plan would not come to pass. Whether she dies, or is imprisoned, or something else. I could only see an genuine DESTROY THE WORLD! scenario in a standalone game. You can shake up the status quo, but probably not too much.
Yeah. That's my speculative fanfiction for today.