But like I said, I shall try to keep opinions quiet until I see her in action.
If only other people would follow your example
In any case, I had a chance to try her out on PBE, some of my (relevant) initial impressions:
Her passive is indeed very powerful. It can be a little tricky to use, but her Q generally will solve that problem, if not you seem to be able to zone people with the threat of it (especially since once you proc the passive, it comes up again about a second or two later, and, with the movement speed on your passive, you can proc it again). How effective it is in an actual game really depends on how well your opponent can guard that weak point; overall very cool and fun mechanic, maybe a little overtuned atm.
Her heal on ult is pretty interesting; since most people don't seem to know/didn't bother to read the announcement/watch the skill video, the heal procs if she hits all four weak points or if she's hit at least one weak point and the target dies, meaning it can also serve as a beacon for your team to focus a particular target. It's very powerful, but in a 1v1 scenario basically boils down to either a heal after you kill your opponent; I don't think hitting all four will be a likely scenario unless you massively outplay your opponent (I could be absolutely wrong though, given the dash, slows, and speed buffs on her kit). Her ult also doesn't have a terribly long duration so you have to either be super mechanically savvy or again, massively outplay your opponent to hit all four. Most likely I see Fiora trading/poking her opponent to half and then all-inning them and just using her ult as a passive refresh and proccing maybe two of the weak points for the kill.
Regarding tank Fiora; it's highly possible, but I don't think she has the utility that Ekko does (nor do I think tank Ekko is even that good, especially compared to champs like Gragas or Rek'Sai or an actual tank champion). I do think more bruiser-ish builds will become common on her since she's less of an assassin now. You have to have pretty good mechanics (or just be completely reliant on your Q) to hit your passive on an intelligent opponent; there are a lot of cases where I started an auto attack that would have hit a weak point, but my opponent moved at an ever so slight angle to make my auto miss the weak spot, or just be at point-blank range where it was a bit more difficult to judge the angle between the champions to make the proper auto attack.