Why I think Fiora's new ultimate fits her theme relatively well.

Envision this. She's against her arch-nemesis in a duel. Both of them have injured each other near the point of death. Fiora realizes that she needs but a few more strikes to finish her opponent off, but her opponent is plotting something big to finish her off before she can even manage those last hits. Her opponent strikes swiftly; so swiftly she cannot counter, and she falls to the ground, dropping her sword and nearly passing out from pain. She begins to watch as her reputation fades. The crowd watches, in absolute horror as the "grand duelist" looks to fade away. However, something sparks in her. She stands back up, appearing completely unfazed from what should have been enough to kill her. She develops a look of anger; an expression that said "You don't believe I would give up that easily, now do you?" She smiles creepily, and lunges forward, quickly striking four times in the their most vulnerable parts, of which she learned about back when she used to practice dueling with her father. The opponent screams in wincing pain and falls to the floor. As they near death, Fiora aims her sword directly at their neck, and says "Always one step ahead." They then pass out. Fiora looks around her, and realizes the crowd is in complete disbelief after what they just witnessed. Not even three seconds later, she hears them all cheer for her. She had won. The crowd goes absolutely wild, and a spotlight shines right where she is standing. In awe, she strikes an awesome pose and remains there. Somehow, she feels recovered; as if she could fight a thousand times more and still have the energy to continue...

When I think of her ultimate, I imagine this. She just barely scrapes a victory versus her opponent, and when she does, everyone cheers her on, and thus she feels better. This is why I believe her ultimate is thematically cohesive and does not need modifications.