My opinion on how to give Fiora purpose ^^
|Duelist||
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| Satisfaction : Fiora restores 3% of her missing health on kills or assists. Duelist's Heart : Fiora gains bonus health equal to 10% of all physical damage she deals against enemy champions for 6 seconds, the amount gained is lost over 3 seconds afterwards. Maximum 400% of her total AD.
Or at least something similar. I don't know how to balance this =)
Here's the explaination:
What is Fiora ? What's she supposed to be ? An assassin ? A fighter ? A melee marksman ? Well, she's certainly the worst at all three.
She doesn't have CC, nor escape abilities, her damage isn't higher than anyone elses, her passive is moderate, her utility non-existent.
But, if you gave her the option to go full AD, assassinate a squishy target with her double-dash-n'-slash-ult-butterfly-combo and come back out as a brutal ass fighter, then she'd be a unique fighter with a very specific role and much more usefulness.
However, like I said, I don't know how to balance this, I'd leave that to Riot.