And make it OP. Only rule is that you can't just use really big numbers. Be creative.

Akali's old passive: 15% of your AP is converted into AD. 15% of your AD is converted into AP. This does not stack with itself.

Subtle change: Now it does stack with itself. Infinite AD/AP anyone?

GO!

Garen passive: Not dropped by any damage.

MF ult: you become immune to CC while channeling it

Karthus ult: Instant cast, no startup time.

Like this?

Shaco: Clones can use Hallucinate now too.

Veigar's primordial burst steals 80% of enemy's AP when cast instead.

Darius: Passive no longer has a stack limit.

.... op ult to the MAX!!!!

Teemo: Shrooms can now spread. Think creep tumors from starcraft.

-=Graves=-
Old Passive: Gains armor and magic resist passively during combat.
New Passive!: Gains armor and magic resist persistently out of combat.

-=Ezreal=-
Old Passive: Whenever Ezreal hits any target with his abilities he gains 10% attack speed for 6 seconds (Stacks 5 times).
New Passive: Ezreal gains 10% attack speed every time he attacks or uses an ability; stacks infinitely.

Kennen - You only need 1 Mark of the Storm to stun (perma-stunned)

[QUOTE=Granodd;29626768]-=Graves=-
Old Passive: Gains armor and magic resist passively during combat.
New Passive!: Gains armor and magic resist persistently out of combat.
[QUOTE]

I actually used to think that was how his passive worked. Summoner level 5 and I'm like "afk in fountain for 10 minutes and become invincible!"

Karthus: Ult now applies to minions and jungle creeps. Split push/counter jungle OP.

Karthus passive now refreshes all cooldowns upon death, in addition to being able to freely cast spells for 7 seconds after death.