How i would rework her,
Passive Expert Black Smith: Poppy can spend 15% of an items gold Value to increase any AD or AP it provides by 10% and any HP Armor or Magic resist it provides by 20%.
NOTE: Poppy would have lower then average base stats(20-40 lower) at lvl18 but using this would have the highest total stats out side of infinite scaling champs like Sion and Veigar, keeping her late game hyper carry status and letting her Black Smithing back ground shine, it favors tankyness as well, and makes raw stats appealing rivals unique passives,
Q Devastating Blow: Poppy slowly stores charges, Hitting an enemy with a basic attack lowers the charge time on the stacks by 0.5 seconds, when activated Poppy Consumes all stacks to enhance her next basic attack, 1 Stack: Poppy Ignores a percentage of the enemies armor, 2 Stacks: Poppy deals Bonus Damage based on a % of the enemies Max HP, 3 Stacks: Poppy knocks the enemy into the air,
W Paragon of Fortitude: Poppy gains a stack each time she is hit with a basic attack she gains a fortitude stack, at 3 stacks Poppy will Gain bonus movement speed towards the attacker who triggered it and either A: Block 50% of the Damage from a melee attack and shield bash the enemy, dealing damage based on her armor, or B: Completely Negate A ranged attack's damage and all on hit effects, 3 second cooldown before new stacks can be acquired after it activates,
E Heroic Charge: Poppy charges at an enemy and carries them further. The initial impact deals a small amount of damage, and if they collide with terrain, her target will take a high amount of damage and be stunned. If used on an Air Born enemy Poppy will slam them into the ground at the end of the charge dealing the bonus damage even without hitting terrain but not stunning them,
R Never Again: Poppy deals Damage and knocks back enemies in a cone, the damage and size of the cone are based on the condition of her allies, lower ally HP will result in a larger cone more damage and a longer knock back, ranging from 100 range to 1000 range, knock back is half the range of the cone,