Name 1 change to 1 ability that would completely break a champion.
Make it is disgusting as possible. I'll start us off.
The first auto after Tumble is a guaranteed crit
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Hook goes through minions
Hook goes through minions
Hook goes through minions
Dark Spheres no longer expire.
- Damnation: Gives AD instead of AP.
Enemies can hear his laugh while he is invisible.
(PLEASE, Riot! It would completely revolutionize Teemo gameplay!)
Ult refund 20% cooldown per champion hit.
To try and be a bit creative with little changes that would be super strong:
Wall is a circle. No dash? No life.
His shield blocks projectiles. Not the ability, the shield.
Shield lasted 3 seconds (Longer than passive channel)
Couldn't be blinded.
and 
: Invisible during Nocturne ult. (sorry)
Anyways on to actually disgusting ability buffs.:
Abilities AOE scale with champ size.
Ult acts like Azir ult.
Instead of stacks he gains AD.
Passive lasts until he respawns.
E applied on hits.
Auto attacks charge passive.
Ult keeps going till you cancel it
http://i1042.photobucket.com/albums/b430/bluemoonwolf8/gifs/15rf2s_zpsl3wqpst7.gif
Q now works on champions.
orbiting stars pull enemies in their range
Because you know.. gravity and such.
no mana usage
Zilean rewind reduced its own cool down by 100% and was free to cast
stacks add up between games
Guaranteed crit after stealth...oh wait...
Passive now also grants permanent ghosting.
Ult now creates an arena to trap all enemies inside.
Death Ray actually causes instant death.
Passive exempts Hecarim from the soft move speed cap.
WHEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
Gets a free auto attack while his ult is channeling
Ulti's knockback has the same range as poppy's ult
The ult now bounces between enemy champions, up to 5 times.
Passive AD boost triggers after applying 1 bleed stack, not 5.
-
Ekko's Parallel Convergence is now instant.
-
Urgot's lock-on doesn't expire until you hit another target with Noxian Corrosive Charge
E charge now always stuns. Always.
Her power chord buffs stack. 3rd Q gives permanent bonus damage, 3rd W gives permanent damage reduction, 3rd E gives permanent movespeed.
Tornado can channel forever, with the tornado becoming faster and stronger every second.
Q lasts till morgana decides otherwise.
Can use the lantern to bonk enemies, dealing souls amount of true damage.
Charm lasts until enemy champion is literally on top of ahri
Daisy is coded as a champion
W tears a hole in reality and can hit any champion on the rift.
Q damage changed to true damage.
his AIDS decreases your resistances. Yes, it stacks and your resistance can go into negative.
His ult is his passive
Fus Roh Dah has global range.
Quintuple crit chance with no crit damage reduction. 
is now 100% crit, bitch.
ult = 
ult + ult
Damage on ult is changed from AP scaling to AD scaling
Love Tap is a critical strike, not simply bonus damage.
Put those crit damage runes to good use.
Sion ult can go through structures.
Or, more subtle, has no global warning.
Stand United (R)
SHIELD: 250 / 550 / 850 (+ 135% Maximum HP)
Ult Costs no mana. Oh wait. :^)
Ult cannot be interrupted by cc.
OR
Ult is now width of Nexus Siege Obliterator.
I'm not sorry.
Eve: q cooldown is instantly refreshed upon hitting a target.
Ult is now a toggle effect
R : gets a real weapon.
10 stacks per Q. Double for cannon minions, champions and large jungle monsters.
W: Pick a card: Enemies can't see you shuffling