Man, what gets me about Aether/Furina fic is the sheer potential for identity-driven angst. Think about it: Furina spent centuries performing a role, hiding her true powerless self behind this extravagant facade. Aether's been on this endless journey, seen countless worlds, but he's also got this core loneliness and that mysterious sibling-shaped hole in his life. Their conflict isn't about petty jealousy. It's about two people who are fundamentally performers—one for an audience, one for a quest—trying to figure out if they can ever take off the mask for someone else. Does he love the flamboyant 'Archon' or the exhausted girl underneath? Can she trust someone whose own past is a riddle? That's way more interesting than standard romance drama.
I've read fics that play with the idea of Aether seeing right through her act from the start, which flips the dynamic. Suddenly her performance is for nothing, and the conflict becomes about her feeling exposed and vulnerable against her will. Other writers focus on Aether's weariness; he's tired of solving everyone's problems, but Furina's is the one performance he can't help but watch, and maybe he doesn't want to 'solve' her, just understand her. The emotional core is trust, but it's trust built on a foundation of shared artifice. It makes for a strangely tender kind of story where the big romantic gesture isn't a confession, it's just silence where neither of them has to pretend.