I have played her (admittedly against AI, only way to make sure I got her, and also admittedly after I typed all that) and I stand by it, albeit for slightly different reasons which may be enough to make me wrong.
It feels like it could be very snowbally in terms of lane dominance. If she gets ahead, unless the mark is behind you, she has a good shot at hitting it. (It IS harder to hit than I thought, but not so hard as to be pointless) And hitting it once can be enough to win a trade, because she also has her abilities AND the mark will be back soon, Don't forget proccing the mark heals her too (a small amount, but still a heal).
I just fear that, if someone gets behind against new FIora, they have no real recourse against her passive except blind luck or not farming unless under tower.
On the other hand, a Fiora that falls behind will have a hard time even proccing the obvios one in their front, out of fear that it start a fight she loses. And if that's the case, I suppose punishing her farther for then not hitting her passive may be cruel.
I was thinking that it would be interesting if autoing the wrong place just triggered it's 2-3 second location change. But then I realized that could be exploited to get the mark to a more profitable location. I still feel like the enemy should be rewarded if they "dodge" fiora's passive, like with Cass's ult, but i'm not sure of a good way to do it. :-/