I've given this way too much thought:
Goth magical girl is the younger sister of the leader of her particular squad of girl power heroes and puts up with being dragged along for the heroics.
But the setting is more of a wild-west-themed fantasy world, and has the magical equivalent of Digimon (perhaps they are fey creatures instead of digital creatures)
Their adventures lead them to needing to conduct a raid against the vault of the biggest church of the lands, where they fight off a powerful (but misguided) young priest.
One of the items they successfully raid is a scroll with a prophecy that seems to indicate that the heroic older sister is supposed to fall in love with that same young priest and have a child who will be the final key to saving the world against [Threat]. But the two of them hate each other now.
Twist: Goth girl thinks that priest boy is kind of hot.
I also cross-posted this idea and thread to Royal Road here.
I'm going to second @[email protected] here in that you should just write what you want to write.
Coming from a fiction writing perspective, it would be "write the story you want to read, because no one else is writing it." This is advice that has been floating around for a while, I forget who first said it.
Writing because you feel you should, will never feel as good as writing what you want to write.