Make one seemingly small change to the game that would create copious amounts of chaos in the game.
I'll get us started:
#There is no longer a CDR cap.
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Armor items are actually useful
Second Lee Sin Q places you behind the target, much like Warwick Q
Ahri has... get this... EIGHT tails. All madness ensues.
Towers actually do damage.
Lucian is white
Amumu has a friend.
Mages autos scale 1:1 with ap.
Going to suggest a few, aiming for the smallest changes I can.
--Auto-attacks no longer stop movement. --Item "Unique" effects don't exist: all item effects stack.
And the big one...That seems small, but forever changes any lane dynamic and most trading situations in bot lane especially...
--Critical hits are predicted one attack ahead of time, and your champion glows (ala Lichbane or other on-hit effect) when your next hit will critically strike.
tentacles can crit now
Movement commands cost 1 mana.
The game obeys laws of physics.
ex. if 
kicks 
lee is the one knocked away and receives more damage than naut does. Or 
trap cannot snare 
cuz he's flying. Or if a melee 
uses Q but you move out of reach of his sword length before the swing drops the attack doesn't hit. The list could go on.
Karthus : Hitting an enemy with R refreshes R.
GG, games are pointless.
No Movement speed cap.
's ultimate "finish line" is actually based on a random part of the map and doesn't represent the actual safe zone.
change the camera angle by 3 degrees.
eve's stealth is aoe for allies
All skillshots become point and click, while all point and click become skillshots.
Oranges no longer have a cast time.
Cast Times no longer exist.
has her bra size reduced to AA.
Minions spawn at 0:30 instead of 1:15. Chaos.
CS deaths reward gold in an aoe, splitting the gold if there are multiple champions nearby. Same as exp distribution.
Everyone's sight radius gets cut in half, wards included
Remove all Boots
Indirectly permaban cass
People get Ranked matches that are actually balanced. :3
Teemo shrooms now execute targets below 50 percent health.
Minions get 10 extra armor.
No attack speed cap.
All auto attacks are now skillshots.
All abilities can now crit.
Kindred's Q applies on-hit effects.
Jax gets a real weapon.
Every champion leaves behind a 
poison trail, always (yes, even then). That scales with both ad and ap. And damages allies, enemies and creeps alike.
Dragon now roams after being hit once (still regens HP while not in combat)
your sight in fog of war gets halved.
when dragon is alive, it attacks everyone in teamfights
when baron and dragon is alive and one of them gets attacked, baron/dragon comes to defend(lmao infernal fixed)
minimap has a 3 second delay
if one disconnects, game ends
turrets have aspd of like 4 times in a sec, same dmg than now
any activatable item you buy activates randomly if a champion/turret is near you
any animal-related champion has a nest somewhere in jungle which is his 'base'
in aram you could actually fall off the edge
in que have an ingame option to play super smash bros melee with other people in que and notice its actually funnier than league itself