So can MF's E be changed into something more useful? Like an ability to help cover a weakness?
There was a time when a slow like that was truly powerful...that was a time when Riot was sensible and didn't think giving everyone and their mother 1-2 gap closers was a good idea and thought to give ranged champions their own gap-closers as well.
In it's current state, her E costs way more mana than what it offers, the cast animation itself makes it an inadequate escape tool except to aid her allies in escaping while you yourself are in a safe position. As a scouting too, well...mana cost and the fact that it's range puts her in range of anyone who is waiting in ambush anyways as I happily learned against an unlucky MF who was trying to scout my bush. It's damage is negligible as no one ever eats the entire duration and overall...it's utility has been diminished by this point in the game.
What was her E's function? To offer a tiny bit of magic damage and hamper the movement of her enemies. In practice, that slow sure won't do anything to a Lucian dash, Yi Alpha Strike, Kha'zix Leap, or a Talon Throatjob, or all the myriad ways champions easily bypass a slow that forces the caster to stand in place to cast. So how does this sound...
Make it Rain: It still does the underwhelmingly shitty damage it always does...but now it also prevents movements other than basic movements.
(or to put it in another way, the mobility portion of leaps and dashes no longer work, 
and 
still does AoE damage around them as though they leaped in place, 
still gets that insane AS buff, 
still have the enemy stunned, 
is still going to get her shield, but they need to WALK to that 550 ranged ADC who just sat there to play that drawn out casting animation now!)
To balance it out, the slow is either reduced or taken out entirely. Even it's AoE can be reduced if it proves too powerful. Oh...and for balance sake, I guess Flash can bypass this.
As for MF as a whole...550 range with 0 self-peels and 0 mobility. To put this into context, I had an easier time killing MF than I did Jinx when I played as Zac despite his heal-reduction.
That or...........perhaps let it scale off AD now because why the hell not, it's not like Riot is a stranger to reckless experimentation without a second thought. (
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