I'd like the idea for there to be a mix up between dealing overall damage to the entire enemy team in small skirmishes that don't really amount to anything except to add more pressure, hence the idea behind Love Tap, and something to do when one particular person is becoming a problem. The Love Tap could still go off AD, as a way to allow for her to focus on that primarily and encourage trying to spread the 'love' around, but the magic damage of the Impure Shot stacking mechanic could scale off of AP, as a way to indirectly mess with tanks or bruisers who would stack armor without giving Miss fortune something like Armor Penetration.
Of course it should take a few shots to ramp up in damage, so that it doesn't become 'better' per say to just focus on one target and ignore Love Tapping everyone around Miss Fortune.
It would add more dynamic to the concept of Love Tap in that she is trying to apply pressure everywhere, so that she might try to keep up the focus and the spread as needed, essentially trying to juggle as many opponents as possible with her passive, which sounds like a really cool mini-game, and we all know how Riot love's their mini-game playstyles.
So here is how I would have it.
Love Tap works as it does as it scales off AD and deals bonus physical damage, promoting Miss Fortune to attack new targets, however, attacking a target who had Love Tap applied to them instead builds stacks of Impure Shots against them, dealing AP-scaling magic damage that ramps up. When Miss Fortune applies Love Tap to a new target after building up stacks of Impure Shots, she consumes all those stacks and adds bonus magic damage to the Love Tapped target.
Thus, rather than discouraging Love Tapping, it encourages people to be much more methodical about when and how they Love Tap.
To make this work, you'd likely have to increase how long Miss Fortune must wait before a target she Love Tapped can be tapped again, but I don't mind that at all, and I feel it would make it fun to play her and try to decisively build stacks and then discharge them, like a sort of Stattik Shiv built into her.