Personally I would love Riot to revisit Fiora and give her some love, she's really fun and I would love to see her fit in somewhere.
Duelist - Compared to other sustain passives is very lackluster. At level 18 you get 100 Health over 6 seconds, assuming you hit just champions 4 times. I think if the passive was over a shorter period of time or higher base health back or a combination of both it would be better and would encourage trading and actually dueling.
Lunge - Once Fiora reaches her target she needs to auto attack that target, much like they did for Akali's Shadow Dance.
Riposte - Needs to be changed the Physical Daamge and scale off AD. Concept wise, she's a Duelist that slices up her enemies with her sword, why does she deal Magic Damage when parrying? I think this would also make it more rewarding for using Riposte during duels.
Burst of Speed - This skill is fine, but I think it would nice if it lasted a little longer to ensure she can get kills. I feel like this is Fiora's bread and butter skills because she is an Auto Attack based champion.
Blade Waltz - The skill is great but... There are many situations where you get "carried away" by champions, ie using Blade Waltz on Shyvana as she Dragon's Descent. Where I don't find it too much of a problem, I think it would be interesting if she was able to Silence her initial target. Also removing the channel time on the cast would be great.
The main thing Fiora doesn't have compared to other Melee Carries is she has no CC of any kind. This makes her not viable in Top Lane as most of them have a combination of CC, durability, self buffs, good damage, and sustain. Where Fiora has no CC, no durability, a good self buff (Burst of Speed), mediocre damage even with Blade Waltz, and mediocre sustain.
Well this ended up being a longer response than intended, my bad.
Passive: I agree, it's kind of blah. It' "okay" for Jungling, but it doesn't really effect anything. It would be better if the effect double for ever hit so the longer she stayed in combat the more she will heal.
Lunge: They actually reverted all the "will attempt to AA after reaching target..." mechanic on GCs. Akali doesn't actually AA after Dance any more and it's super annoying and kind of silly. The inter-cd should also be increased by like a fraction of a second. If you are spamming Lunge it's really easy just to use up both in one go.
Riposte: I don't actually mind the Magic Damage, but it is confusing why it scales off AP and not AD (given the fact she fights people with a sword not the reasoning behind why it scales of AP). Though I think it should also reflect a % of damage back on the attacker or you gain something for timing the Ability. Siv gets Mana for timing her Spell Shield which is the Magic version of the Ability, Jax has an AE Stun for a similar function and Black Shield lasts "forever". What I also don't get is apparently Magic Shield and Black Shield block's Dar's Ult but NOT Riposte... What is that all about?
Burst of Speed: I personally find it funny that out of all the "Reset on K/A" Abilities Assisting only reduces the CD by 1/2 where as for Eve, Kat, Dar etc Assisting gives the same reset as Killing.
Blade Waltz: I actually like how this Ability works. Though I kind of wish the Range was increased a bit. I mean she can Lunge for 600 units why can't she BW for the same range?