Perhaps a fanfic where Aatrox, in his more sentient moments, is intrigued by Irelia's unwavering will and spirit. He decides to test her, not as an enemy but as a way to understand the concept of hope that she represents, leading to a series of duels and philosophical exchanges between them.
The story could have Aatrox's ability to drain life force be a key plot point. Maybe he is weakened in the fanfic and needs to absorb life energy to regain his strength. Irelia's ability to control her blades could be used in a more creative way, like using them to form a shield against Aatrox's dark magic or to create a bridge across a dangerous chasm when they are on a quest together.
The appeal also lies in the fact that both are fan - favorite characters. People love to see more of their backstories and how they would react in situations not shown in the game. Writers can expand on their personalities and create unique plotlines that keep readers hooked.
There's also a fanfic called 'Aatrox: The Dark Warrior'. In this one, the author focuses on Aatrox's battles and how he overcomes his enemies. It's full of action and really exciting to read.
The plot might involve Aatrox being intrigued by Fiora's unwavering courage. He could see in her a potential to be a great warrior on his side, not as a minion but as an equal. So he tries to persuade her to join his cause, which leads to a series of intense dialogues and battles between them, as Fiora is hesitant but also curious about Aatrox's power.
One popular 'irelia x riven fanfic' is 'Blades of Bonding'. It beautifully weaves their stories together, exploring their possible relationship in the League of Legends universe. The author does a great job of portraying their personalities and how they might interact.
For Fiora, you can focus on her noble upbringing and her sense of honor. Show how these traits influence her actions when she meets Aatrox. For example, she might be disgusted by Aatrox's use of dark magic but also respect his power.
For Aatrox, you could focus on his past as a Darkin. His internal struggle between his dark nature and any glimmers of his former self could be explored. Show how Fiora's presence either exacerbates or soothes this conflict. For Fiora, her pride as a duelist is key. You can show how she reacts when faced with Aatrox's power that might be greater than what she's used to. Does she become more determined or is she momentarily daunted?
One idea could be a story where Aatrox, the Darkin Blade, and Fiora, the Grand Duelist, are forced to team up. Maybe there's a greater threat to their world that only their combined skills can face. They start off distrusting each other, but as they face challenges together, they gradually come to respect one another's abilities.