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

Annie Annie
  • New secondary attribute: Support.
Alistar Alistar
  • Primary attribute changed to Support from Tank.
  • New secondary attribute: Tank, Fighter.
  • Unbreakable Will.png Unbreakable Will
Ashe Ashe
Blitzcrank Blitzcrank
  • Primary attribute changed to Support from Tank.
  • Secondary attribute changed to Tank, Fighter from Fighter.
  • Rocket Grab.png 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.png 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.
Brand Brand
  • Sear.png Sear
    • Stun duration changed to 1.8/1.9/2.0/2.1/2.2 seconds from 2 seconds at all ranks.
Caitlyn Caitlyn
Corki Corki
  • New secondary attribute: Mage.
  • Missile Barrage.png Missile Barrage
    • Cooldown decreased to 1.5/1.3/1.1 seconds from 2 seconds.
    • AD ratio changed to Bonus AD Ratio.
Darius Darius
Dr. Mundo Dr. Mundo
  • Infected Cleaver.png Infected Cleaver
    • Tooltip now shows the max. damage against monsters (300/400/500/600/700)
Draven Draven
  • Spinning Axe.png Spinning Axe
    • AD Ratio decreased to 40/50/60/70/80% AD from 45/55/65/75/85% AD.
  • Blood Rush.png Blood Rush
    • Now only reduces the cooldown by 6 seconds when he catches a Spinning Axe.png Spinning Axe.
Elise Elise
  • Secondary attribute changed to Fighter, Support from Fighter.
  • Cocoon.png Cocoon
    • Cooldown increased to 16/15/14/13/12 seconds from 14/13/12/11/10 seconds.
Evelynn Evelynn
  • Secondary attribute changed to Mage, Fighter from Mage.
  • Hate Spike.png Hate Spike
    • AP Ratio decreased to 40% from 45%.
Ezreal Ezreal
  • Secondary attribute changed to Assassin, Mage from Mage.
  • Essence Flux.png Essence Flux
    • Can now damage enemy minions and monsters, but the base damage is halved against them.
  • Arcane Shift.png Arcane Shift
    • Now prioritizes champions.
Fiddlesticks Fiddlesticks
Fiora 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.png Burst of Speed
    • Attack speed bonus reduced to 40/60/80/100/120% from 60/75/90/105/120%.
Fizz Fizz
  • Trickster.png Trickster
    • Damage decreased to 60/100/140/180/220 from 70/120/170/220/270.
Garen Garen
  • Tooltip Update.
  • The quote "DEMACIA!!!" is now for Judgment.png Judgment instead of Courage.png Courage.
Kassadin Kassadin
  • Riftwalk.png Riftwalk
    • Cooldown increased to 130/115/100 seconds from 7/6/5 seconds.
    • Now uses a system similar to old Noxian Guillotine.png Noxian Guillotine. If Riftwalk.png 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.png Riftwalk in the time specified above, it will go on its full cooldown.
Master Yi Master Yi
  • Minor kit rework.
  • Secondary attribute changed to Fighter, Mage from Fighter.
  • Alpha Strike.png 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.png Meditate
    • Damage reduction changed to 40/45/50/55/60% (+ 5% AP) from 50/55/60/65/70%.
  • Wuju Style.png 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.png 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.png Alpha Strike: If it kills an enemy champion, his next Alpha Strike.png Alpha Strike in 3 seconds cost no mana.
      • Meditate.png Meditate: Damage reduction also scales off 5% of his total AD.
      • Wuju Style.png Wuju Style: Reusing this ability will stack its true damage up to 2/3/4 stacks.
Vladimir Vladimir
  • Tooltip update.
Riven Riven
Zed Zed
  • Contempt for the Weak.png Contempt for the Weak
    • Damage reduced to 4/5/6% of their maximum health.
    • Now deals true damage.
  • Death Mark.png Death Mark
    • No longer detonates if Zed dies before detonates.

Easter Eggs

Mordekaiser 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.png Children of the Grave or Siphon of Destruction.png Siphon of Destruction acitve kills the target)
  • New taunt only in Brazilian server: "HUEHUEHUEHUEHUEHUEHUEHUEHUEHUEHUEHUEHUEHUE."
Vi Vi
  • Her Vault Breaker.png Vault Breaker is occasionally referred as 'Fail Train' while channeling on the channel bar.