Break a champion by reversing the passive and active portion of an ability
Passive: Jax gains 4/7/10+10%BAD armor and 4/7/10+20%AP magic resistance.
Active: Jax deals an additional 35/55/75+25%AP magic damage on every basic attack for 8 seconds.
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Ragnarok
Passive: Olaf is immune to CC effects and gains 40/60/80 AD.
Active: Olaf gains 10/20/30 armor and magic resist.
Dunno how to top that.
: Tag Team/Skystrike
MUAHAHAHA! I guess it's not quite the same as you wanted, but I want to swap the forms anyway.
Passive: Shoots out random hawks to reveal pretty much everything around her within a range of 2500 / 3250 / 4000 / 4750 / 5500
Active: Ashe gains bonus gold for killing a minion or structure for the next X Seconds.
Wards? Who needs wards!
Astral Infusion
Passive: Each enemy hit by Starcall heals the lowest nearby ally for 25/35/45/55/65(+40%AP), +1% per 1% missing.
Active: Soraka heals herself for 120/150/180/210/240(+60%AP)
Impure Shots
Passive: Miss Fortune gains +20/30/40/50/60% AS and her attacks apply Grievous Wounds for 2 seconds
Active: Miss Fortune's attacks deal 6% AD magic damage for 6 seconds, stacking up to 5(+ Bullet Time ranks) times.
Passive: Fiora parries basic attacks and reflects 60/110/160/210/260 (+100% Ability Power) magic damage back to the attacker. Works against champions, large monsters, and large minions.
Active: Fiora's Attack Damage is increased by 15/20/25/30/35
...
If only she had at least one or two more AP ratios to make an off-build possible.
Passive: Akali becomes stealthed and briefly gains bonus movement speed, which refreshes each time she re-enters stealth. If she uses a basic attack or an ability while stealthed, she will briefly reveal herself for 0.65 seconds.
Active:Akali's basic attacks deal 6% (+1% per 6 AP) of her attack damage as bonus magic damage.
Akali has a natural 6% (+1% per 6 bonus AD) spell vamp.
Passive: Deals 30/50/70/90/110 (+20% Ability Power) magic damage to nearby enemies each second. Active: Drains 30/42/54/66/78 Mana to restore 20/27/34/41/48 Mana when Karthus kills a unit.
His active now sucks, but dat passive...
Heartseaker Strike
passive: Deals 80/140/200/260/320+360%BAD physical damage to all enemies in front of Pantheon over 0.75 seconds every time either his basic attacks of Spear Shots critical strike.
Active: Pantheon causes his Spear Shots and basic attacks to critically strike for 6 seconds.
Rat-Ta-Tat-Tat
Passive:
Twitch's auto-attacks pierce his target dealing damage to units along the line of fire. Also, Twitch's auto-attack range is 850.
Active: Twitch inflicts Deadly Venom on units he deals damage to through auto-attacks or abilities. Deadly Venom deals true damage over time.
Idunno if that would break him or make 
his main build.
Blitzcrank: Passively deals AoE damage and silences nearby enemies. Active: Kennen ult
Active: Zed's attack damage is increased by a percentage.
Passive: Zed's shadow dashes forward, remaining in place for 4 seconds. Reactivating Living Shadow will cause Zed to swap places with this shadow.