Master Armorsmith:
This champion doesn't have Base Armor nor does he gain Armor/lvl but this champions Armor obtained through Items, Runes&Masteries and Ingame buffs is multiplied by 2.
So, Thornmail alone gives him 200 Armor.
Xtremey7 votesINTENT: Yasuo's critical strike chance is doubled, but the damage dealt by his critical strikes is reduced by 10% (25% on Steel Tempest).
RESOLVE: At maximum Flow, the next instance of damage coming from a champion or monster against Yasuo will first convert his Flow into an absorption shield. After 1 second, if not already depleted, the shield dissipates and Yasuo loses all his Flow
Xx420n0sc0p3sxX1 votesLol'd
Orbv4 votesHear No Evil, See No Evil, Speak No Evil:
When this champion is damaged by an enemy champion, that champion has a 30% chance to lose sight (blinded or shrouded in darkness), 30% chance to be silenced, and 30% chance to lose hearing (static sound in the background, or a maniacal champion laugh on repeated) for 1.25 seconds. This cannot be activated onto a champion more than once every 10 seconds.
BouncingPecs2 votesThis would be good on Wukong, but now we need another monkey champion.
OR
We could have a champion who is full of "bad luck". With abilities like The UnLucky Rabbits Foot or Shatters Glass or Salt Spiller
Lord Dusteon3 votes"Draven's critical strikes deal bonus physical damage over time. Spinning Axe also causes this effect even if it does not critically strike."
Agidyne2 votesrip wicked blades
The Whamboozler1 votesPity it was his huge damage that made him viable really. There's a reason he don't see much play now. Or did you think the question was "Create a passive essential to a champion staying relevant"?
SouL1ess2 votesProjectiles are annoying: your resistances are doubled against any attack or spell that involves a projectile. In addition, you cannot receive critical strikes from ranged autoattacks.
Xx420n0sc0p3sxX1 votesI actually want to see how that passive would pan out, it would be like the worst early game against AD champs, but late game you would be the greatest tank......for AD.
Lord Macaroni1 votes
ActualPain1 votesEyes of Pain: Every time you look at an enemy champion, he/she dies.
7ha7guy7771 votesDeath Ignored
Upon death, this champion regains 100% of its hp over 1 second. (there is no cooldown)
Bears Dont Care1 votesHadoken: Each time you use an ability, your character proceeds to fire a projectile every second, for the next 10 seconds, dealing 200 + (100% AP) in the direction of the enemy champion. Range is 2000, projectile speed is 2000.
Heheh....
MyGhettoFantasy1 votes"Lolzyoumadbrah?": Everytime you kill a champion their computer shuts off.
TheI3igDaddy1 votes
A Slime Appears1 votesMaster Armorsmith:
This champion doesn't have Base Armor nor does he gain Armor/lvl but this champions Armor obtained through Items, Runes&Masteries and Ingame buffs is multiplied by 2.
So, Thornmail alone gives him 200 Armor.
Eh, a champion with that passive would suck. They'd get harassed out of lane hilariously easy and the new jungle would curbstomp them, unless they had some other passives or stupid OP QWE.
iainB851 votesINNATE: This champion cannot cancel their basic attacks and they will miss if they loses sight of their target before they hit. If this champion enters a movement command while winding their basic attack or this champion's Q ability, they will drive through that direction when they launches their attack, dashing a short distance.
Scorpoison1 votesEvery other basic attack has a 50 % chance to inflict concussive damage, an enemy hit with concussive damage 3 times within 10 seconds is dealt 50% of their current health as physical damage. Concussive damage can stack on multiple enemies and is applied by 
The Whamboozler1 votesFilthy Rich: This champion starts with +300 gold and gains 150% gold for kills and assists.
Stubborn: This champion ignores the first CC effect that strikes him. This ability cannot activate more than once every 10 seconds.
Vicious Haggler: This champion purchases items at 30% less gold cost.
Schadenfreude: Any damage taken by nearby enemy champions heals this champion for 15% of the damage taken.