Here's a rough idea of what I think could be a kit for Fiora:
General: Probably nerfs to Fiora's early game damage (base AD, base attack speed) to allow better scaling on her abilities.
Passive - Master Fencer: Fiora's basic attacks deal 2% of her target's maximum health in physical damage, stacking up to 4 times. Stacks last up to 5 seconds.
Q - Lunge: Fiora dashes forth 600 units, stopping at the first enemy unit hit and dealing 10/20/30/40/50 (+ 100% AD) physical damage. Lunge applies on-hit effects and can critically strike, and its dash speed increases with Fiora's movement speed. Costs 50 mana. 8/7/6/5/4 second cooldown.
W1 - Parry: Fiora becomes immune to spells and basic attacks for the next 0.5 seconds. If she blocks at least one spell or basic attack from a champion or large monster, she can cast Riposte within the next second. No mana cost. 9/8/7/6/5 second cooldown.
W2 - Riposte: Fiora's next basic attack Disarms her target for 1 second, rendering them unable to attack. Disarm is reduced by Tenacity, and Riposte resets Fiora's basic attack timer.
E - Flèche: Fiora channels for 1 second, during which she strides towards the target area, attacking three times in front of her as she travels. Each attack deals 40/45/50/55/60% AD true damage, applies on-hit effects to the first enemy hit, and can critically strike for 50% extra damage. Blade Waltz's maximum range increases with Fiora's movement speed, and the channel completes 0.5% faster for every 1% bonus attack speed, up to a minimum of 0.5 seconds (the distance traveled and number of attacks remains the same) at 100% bonus attack speed. Costs 60 mana. 15/13/11/9/7 second cooldown.
R - Duel: Fiora challenges the target enemy champion to a Duel. During a Duel, Lunge and Flèche have their cooldown reduced by 75% if they damage her target, and Riposte will cause her to dash if used on the target, becoming untargetable in the process. Basic attacks score 1 point, and critical strikes score 2 points, and at 10 points Fiora ends the Duel with a coup de grace, dealing 250/375/500 (+ 125% bonus AD) magic damage, increasing by 1% for every 1% of the target's missing health for a maximum of 500/750/1000 (+ 250% bonus AD) magic damage. If Fiora does not land a basic attack against her target within 5 seconds when dueling them, the Duel is canceled. Costs 100 mana. 130/90/50 second cooldown.
Basically, the idea here is to turn her into an ability-based melee carry who commits to a single target (as a duelist should) and uses her abilities strategically for either damage or utility/mobility. She'd get small windows where she'd get to dodge or stop damage (Parry/Riposte), but overall she'd also give her opponent tons of opportunities to outplay her: thematically, she's really strong if you follow the rules of fencing with her (e.g. commit to a single target, attack, parry and press your opponent at the right moments), but she's bound to those rules, so as her opponent if you violate them (such as by stepping outside of her "piste" while she's channeling Blade Waltz, which is illegal in fencing, or just declining her challenge when she Duels you by keeping your distance) you'll gain a significant edge over her.
's current kit is pretty nice, or at least has a great core concept (Lunge is fun, the idea of parrying an attack is super sweet), but a lot of her could be replaced with much more interesting abilities. Her passive isn't particularly cool or useful to her role/theme, and her ult is just not fun to deal with (her E is sort of okay, but I'm not even sure if it needs to work as an active ability). I think Fiora needs to have a kit much more focused on moment-to-moment reaction and capitalization, so something closer to real fencing, and I think that would mean making her more cast-focused in a style similar to 
, where each ability is a small parcel of power on a fairly short cooldown that can become really impactful if used at the right moment.
