Writing 'Wriothesley x Lumine' stuff that actually hits hard emotionally is tricky. A lot of fanworks fixate on either the grumpy/sunshine dynamic or the 'he's a dragon, she's an outlander' power fantasy, and those can be fun, but they leave the deeper stuff untouched. What makes Lumine compelling is her journey across Teyvat—the search, the loss, the weight of witnessing centuries of stories. The emotional core might be less about explosive romance and more about two people who fundamentally understand loss finding a quiet harbor in each other.
I'd suggest playing with timeline and memory. Maybe Wriothesley, with his own long history and possibly non-human lifespan, recognizes a kind of exhaustion in her that others don't. He's seen a lot too. A scene where they're just sitting in his office, not talking about the big mission, but about some small, centuries-old tradition from her travels that's since vanished. Him offering her a cup of tea that reminds her of a flavor from a forgotten nation. It's in those tiny, specific moments of shared, unspoken history that the connection feels earned, not forced.
The physicality of his dragon traits shouldn't just be for show. Is his temperature regulation off when he's stressed? Does Lumine notice a subtle shift in the warmth of a room when he's guarding her back in a fight, and what does that silent, instinctual protection mean to someone who's always had to be the protector for her brother? Let the supernatural elements serve the emotional beats, not overshadow them. End on a note where the comfort they find isn't a solution to their larger quests, but simply a reason to keep going.