This is completely inspired by Emptylord's custom patch notes. I will split my ideas into 2 parts: My Own Ideas and Ideas with References. Also, I will copy some amazing ideas into this post (this is incomplete and I will update it over time)

My Own Ideas

General

  • Champion can have multiple secondary attributes instead of only one.

Champions

AnnieSquare Annie
  • New secondary attribute: Support.
AlistarSquare Alistar
  • Primary attribute changed to Support from Tank.
  • New secondary attribute: Tank, Fighter.
  • Unbreakable Will Unbreakable Will
AsheSquare Ashe
BlitzcrankSquare Blitzcrank
  • Primary attribute changed to Support from Tank.
  • Secondary attribute changed to Tank, Fighter from Fighter.
  • Rocket Grab Rocket Grab
    • Cooldown increased to 20 at all ranks from 20/19/18/17/16.
    • Mana cost changed to 130/120/110/100/90 from 120 at all ranks.
  • Static Field Static Field
    • Now cannot hit neutral monsters unless allied units drawed aggro on them. This cannot hit minions if Blitzcrank did not hit it in the last 10 seconds.
BrandSquare Brand
  • Sear Sear
    • Stun duration changed to 1.8/1.9/2.0/2.1/2.2 seconds from 2 seconds at all ranks.
CaitlynSquare Caitlyn
CorkiSquare Corki
  • New secondary attribute: Mage.
  • Missile Barrage Missile Barrage
    • Cooldown decreased to 1.5/1.3/1.1 seconds from 2 seconds.
    • AD ratio changed to Bonus AD Ratio.
DariusSquare Darius
Dr. MundoSquare Dr. Mundo
  • Infected Cleaver Infected Cleaver
    • Tooltip now shows the max. damage against monsters (300/400/500/600/700)
DravenSquare Draven
EliseSquare Elise
  • Secondary attribute changed to Fighter, Support from Fighter.
  • Cocoon Cocoon
    • Cooldown increased to 16/15/14/13/12 seconds from 14/13/12/11/10 seconds.
EvelynnSquare Evelynn
  • Secondary attribute changed to Mage, Fighter from Mage.
  • Hate Spike Hate Spike
    • AP Ratio decreased to 40% from 45%.
EzrealSquare Ezreal
  • Secondary attribute changed to Assassin, Mage from Mage.
  • Essence Flux Essence Flux
    • Can now damage enemy minions and monsters, but the base damage is halved against them.
  • Arcane Shift Arcane Shift
    • Now prioritizes champions.
FiddlesticksSquare Fiddlesticks
FioraSquare Fiora
  • Primary attribute changed to Assassin from Fighter.
  • Secondary attribute changed to Fighter from Assassin.
  • Base AD decreased to 52.5 from 54.5.
  • Burst of Speed Burst of Speed
    • Attack speed bonus reduced to 40/60/80/100/120% from 60/75/90/105/120%.
FizzSquare Fizz
  • Trickster Trickster
    • Damage decreased to 60/100/140/180/220 from 70/120/170/220/270.
GarenSquare Garen
  • Tooltip Update.
  • The quote "DEMACIA!!!" is now for Judgment Judgment instead of Courage Courage.
KassadinSquare Kassadin
  • Riftwalk Riftwalk
    • Cooldown increased to 130/115/100 seconds from 7/6/5 seconds.
    • Now uses a system similar to old Noxian Guillotine Noxian Guillotine. If Riftwalk Riftwalk hits an enemy champion, the cooldown will be decreased to 7/6/5 seconds and can be used again within 12 seconds after previous cast with higher damage and mana cost. If he does not use Riftwalk Riftwalk in the time specified above, it will go on its full cooldown.
Master YiSquare Master Yi
  • Minor kit rework.
  • Secondary attribute changed to Fighter, Mage from Fighter.
  • Alpha Strike Alpha Strike
    • Now also scales with 100% AP.
    • Now deals magic damage if his AP is higher then his AD and cannot critical strike if it deals magic damage.
  • Meditate Meditate
    • Damage reduction changed to 40/45/50/55/60% (+ 5% AP) from 50/55/60/65/70%.
  • Wuju Style Wuju Style
    • Active true damage decreased to 4/6/8/10/12 (+ 10% AD) from 10/15/20/25/30 (+ 10/12.5/15/17.5/20% AD)
  • Highlander Highlander
    • Passive cooldown reduction reduced to 50% from 70%
    • Now resets the cooldown of his other abilities on kill and assists while active and being special effects to his abilities.
      • Alpha Strike Alpha Strike: If it kills an enemy champion, his next Alpha Strike Alpha Strike in 3 seconds cost no mana.
      • Meditate Meditate: Damage reduction also scales off 5% of his total AD.
      • Wuju Style Wuju Style: Reusing this ability will stack its true damage up to 2/3/4 stacks.
VladimirSquare Vladimir
  • Tooltip update.
RivenSquare Riven
ZedSquare Zed
  • Contempt for the Weak Contempt for the Weak
    • Damage reduced to 4/5/6% of their maximum health.
    • Now deals true damage.
  • Death Mark Death Mark
    • No longer detonates if Zed dies before detonates.

Easter Eggs

MordekaiserSquare Mordekaiser
  • Referred as 'Huekaiser' in the chat filter when he is the only surviving champion.
  • New death quote: "I report you."
  • New quote: "Gibe money please." (When Children of the Grave Children of the Grave or Siphon of Destruction Siphon of Destruction acitve kills the target)
  • New taunt only in Brazilian server: "HUEHUEHUEHUEHUEHUEHUEHUEHUEHUEHUEHUEHUEHUE."
ViSquare Vi
  • Her Vault Breaker Vault Breaker is occasionally referred as 'Fail Train' while channeling on the channel bar.