Okay so I've been neck-deep in this ship's fics since like, forever, and the emotional tension thing is like… their entire deal. Rise's whole public-facing pop star persona versus Naoto's quiet, almost closed-off detective shell is just this giant neon sign flashing 'UNRESOLVED FEELINGS' that writers love to pick at. A lot of stories I've seen put them in scenarios where Rise’s natural empathy and expressiveness bumps right up against Naoto's need for order and emotional distance.
What I think a bunch of authors get really right is using the investigation angle, weirdly enough. Like, Naoto gets wrapped up in a case and Rise insists on 'helping,' which usually means she pushes past all the professional boundaries Naoto tries to set up. That's where the tension lives—Rise seeing right through the 'detective prince' act to the person underneath, and Naoto being equal parts unnerved and, secretly, relieved someone finally does. The growth usually comes from Naoto learning to accept that being vulnerable with one person isn't a weakness, and Rise learning that not every emotional moment needs a spotlight or a song; some are just quiet and hers alone.
There's this one fic I can't remember the name of where Rise got a vaguely threatening fan letter, and Naoto offered to look into it professionally. The whole story was just them sitting in Naoto's apartment going over evidence, but the tension was all in the silences and the sideways glances—when Rise would make tea and Naoto would almost forget to say thanks, or when Naoto would slip and use Rise's given name and then get flustered. The growth was super subtle, just Naoto's shoulders losing that stiff line by the end, and Rise talking about her idol stress without performing it. It felt real in a way the game didn't always have time for.
I guess for me, the best fics don't try to force them into a super loud, dramatic romance. The tension is in the small cracks in their armor, and the growth is watching those cracks slowly, carefully widen until they let each other in.