I've always gravitated towards the less obvious dynamic between them, the one that isn't about a sudden romantic spark but about a fundamental incompatibility that forces change. The classic 'Araki Forgot' meme gets flipped on its head—instead of Rohan forgetting he can't use Heaven's Door on Koichi, he remembers, and that's the problem. He's an artist who dissects truth, and here's this utterly sincere, morally rigid kid who is, by his own power's definition, completely opaque to him. It creates this fascinating intellectual itch Rohan can't scratch. Fics that really dig into that, where Rohan's fascination is born from frustration and Koichi's quiet stubbornness becomes a wall even Heaven's Door can't breach, feel more true to their characters than any straightforward fluff. The romance, if it happens, has to grow from that soil of mutual incomprehension turning into a weird respect.
I'm less convinced by the 'time-travel fix-it' or 'college AU' takes, unless they're exceptionally clever. Placing them in a normal setting often sandpapers off the specific edges that make their interaction compelling—the supernatural element, the inherent danger of Rohan's power, the fact Koichi is literally carrying a ghost. The best stories I've read keep those elements central, using Stand battles or bizarre Morioh occurrences as the pressure cooker for their relationship. A trope I stumbled on once that worked surprisingly well was 'body-swap', but not played for laughs. Rohan trapped in Koichi's body, experiencing the weight of Echoes ACT3 and that persistent, low-level anxiety Koichi carries, while Koichi has to navigate Rohan's perceptual overload and the social alienation his power creates. It's a trope that forces the empathy their normal interactions might lack.