Currently Fiora's Passive looks like this:
Duelist: Fiora regenerates 7 + (1 × level) health over 6 seconds each time she deals damage. Striking champions will cause this effect to stack up to 4 times.

Duelist will proc whenever Fiora autoattacks or lands Lunge or Blade Waltz.
Duelist will not proc with the magic damage dealt by Riposte's active.

My suggestion:
Duelist: Fiora regenerates 7 + (1 × level) health over 6 seconds each time she deals damage. Striking champions will cause this effect to stack up to 4 times.

Duelist will proc whenever Fiora autoattacks or damages an enemy.

This change increases Fiora's Sustain through use of Riposte and Item Actives.

Additonally the same change would be applied to Burst of Speed, allowing faster MS stacks.

I feel like this would be nice. Fiora needs to be able to step back up and claim her title of the Grand Duelist. I would actually like to have the chanced to get jacked up by a fiora in 1v1 and be able to say, Oh, that's right, she's made to do that.

How about we make Duelist a not-useless passive?

It's 1-5 HP regen per second, give or take a decimal place, at level 1. At level 18, it's 4-17. AFTER auto attacking. For 6 seconds.

Meanwhile in Nasus' world, one auto attack at level 1 usually begets visible healing numbers, especially with his Q. I mean, I know it'd be ridiculous to ask for Nasus' passive on Fiora, but come on. The bruiser's sustain is 10 times more reliable and at least 3 times as good as the melee carry's.

Remember, in order to stack Duelist, you need to trade. Considering that Duelist is a HoT, if you're going against a bursty champion (or even Singed, for goodness sake), your HPS isn't enough to give you that advantage over your opponent. Most of the time.

Here's an idea: why can't we let Riposte reflect ANYTHING? Sivir and Nocturne both have shields that will block an ability... let's add that feature to Fiora, considering she has absolutely no escape mechanism. Similar characters, such as Yi, Olaf, and Jax, all have escapes or mechanisms that can be used to escape... where's Fiora's? (Especially considering that Fiora's bases are even less tanky than other options... ouch.)

Add a slow, or a stun, and voila , she is FOTM