She's going to be strong, and after a patch or two, and perhaps some competitive play, she'll get her passive true damage nerfed. Maybe they'll give her a full reset on her Q if her Q hits a vital in order to emphasize the dance aspect of Duelist's Dance and shift power to multiple consecutive vital hits over one or two poke hits then going all-in.
She does have a lot of similarities to
now, so I don't see her as not being strong.
That said - her ult is pretty much an either-or decision now. You ult someone low health and pretty much ignore the dance to get the heal, or you ult someone that isn't so you can lay down a lot of damage against them with ~3 true-damage hits. Anyone that isn't a potato won't let you get all 4 hits off.
With the relative ease you have in evading Fiora's true damage hits over just being super-reliable and responsive like Irelia, I don't see Fiora having a good enough showing to merit nerfs.
And I really doubt Riot is going to nerf her true damage if she does, seeing as Fiora's main problem right now is going to be setting up an ideal situation to use it in the first place.