I used to play Ap MF all the time, it was great. I built on hit effect items like wits end and nashors tooth. (as maxing w after e was actually viable)
Now it's changed to a very bad, and boring way to play.
Her q scales with ap, but deals ad, so it's pretty bad. Good for lichbane though, as with c/d it's about 2 seconds.
Her w is pretty much useless, other than being used with your ult for the small ap damage bonus, and to land more q's. (it prioritizes things affected by her w more)
Her e is really her main lane damage source, and it sucks at it's job. I find the increased duration ironically forces her to farm more instead of trade,(Assuming the increased duration was done to reduce e last hitting) and also looks kind of clunky.
Her ult is powerful, but the sea is a cruel mistress, as it also scales with ad.
Now here's my suggestions.
The Q can stay the same (as a lich bane proc tool).
The W can scale with ap.
The Ult should have it's ap scaling removed to compensate for the ap ratio in the w.
The E should either revert the e duration change, or become instant cast like how Katarina's ult was changed. Either starting the damage instantly, the slows instantly or both instantly. (though changing the slow to apply instantly would affect ad mf aswell) If the problem with the e was that it's too good at pushing minion waves... simply make it reduced damage to minions!
With these changes, Ap on hit mf can actually be a really viable pick!
Her w will go well with on hit again, and it wont cause problems with her ult.
The e changes would make her e a better tool for damage and/or for catching people with auto attacks.
One of my favorite parts is that her ults ap scaling will actually deal ap damage (from the w!)
I'll be honest i like the changes for ad mf, but it ruins it for ap mf.
(extra)
One of the very fun (and powerful) ways to lane with on hit mf, was to e them, and while they were slowed, auto attack them as much as possible, (to build up dem stax). It hurt, and in a good scenario, they could hardly fight back at all. All they try to do is escape the circle. If they did fight back, then they would receive the full duration of damage from the e, AND you would simply build more stax. This was very easily counterable though, as if you had any mobility (flash, playfull/trickster,riven-mid,lb, ect) you could easily get out. If you had sustain, she could never get you low enough to finish you off with her ult. Also she's rather squish, and has no real excapes so if someone like xin zhao ganks AND lands an e, she can't fend him off. (though her passive speed and e slow can stop him before he's close enough)
I miss out damaging enemy adc's mid game with your auto attacks, or dealing unexpected damages with your ult and e to finish someone off. It's a really fun build, and i'd really like to see it again :c