So I saw another Fiora rework get suggested, and I thought I'd share my thoughts on the matter. Changes are in red
While Duelist is a useful passive in the laning stage, it scales very weakly into the late game. By changing the health regen to scale off maximum health rather than based on level, it allows for her to be build with more sustainability if required. By reducing the amount of time for the heal to take effect, it encourages her to remain in combat at higher levels to keep the heal working. (The current numbers would reduce the healing effect with no items, but start to increase it with an item such as Frozen Mallet)
One of Fiora's disadvantages when compared to a champion like, say, Jax, is a lack of manoeuvrability, as her only mobile ability revolves around jumping onto an enemy target. By changing this to a non-targeted dash, it allows it to be useful for engaging as well as escaping. Raising the cooldown and requiring the first lunge to land for the second lunge to be usable prevent it being too irritating as an escape method, and encouraging aggressive play.
Riposte's passive is essential to Fiora's early game, and doesn't need any tweaking; in my opinion. The active, though, doesn't seem as good as it could be. With a 1.5 second active time, it can be activated without much thought as to whether an autoattack will land in that time; especially when against champions like Master Yi or Caitlyn. By lowering it to 0.5 seconds, Fiora would have to watch the autoattack animation to time it exactly, which would require more skill and timing; appropriate for a parry. The magic damage done by a successful parry also doesn't fit well with the rest of her kit, and a stun would give her some much needed crowd control. Requiring 175 range for an effective stun also pushes the player to engage other melee characters close up; with the longer cooldown preventing it from being overpowered in a duel.
Burst of Speed is a powerful skill, and its only downside is that the move speed requires a hit to be landed. However, the need to use Lunge to begin chasing with Burst of Speed is a crucial part of her kit, and I don't think needs changing.
Fiora's ultimate offers a great deal of damage to either a single target, or a decent amount of damage to a group of targets. However, in a teamfight it does not assist your allies much, and so I have switched out some of the damage for armor shred.