What if Impure Shots worked less like Vayne and more like Ashe?
Basically, what if, instead of adding a debuff stack to the opponents you hit with Impure Shots, you added a stack onto MF herself? The problem I see with Impure Shots is that, although there's some potential tank busting mixed damage, which would overall help with diversifying herself from other picks and give people a reason to pick her, it's all based on focusing one damn dude down all day. Meanwhile, Ashe can pick up focus stacks by targeting anyone, then just unload them on her preferred target when she has 5 stacks. Why not make it so that as long as MF is actively engaged in the fight, her Impure Shots can ramp up and deal more magic damage without having to be stuck on one target the whole time?
As it stands now, MF's mixed damage doesn't get much done in comparison because Vayne mixes her damage with single-targeted true damage, allowing her to quickly drop one foe and then work on the next one. With a change like this, however, MF could work fights by attempting to get autos off to ramp up, then applying it to enemies more liberally, yet less potently, than Vayne. Thoughts?