Since I am completely out of ideas for any complete custom champion, here is a game mode I thought: might not end up as huge as the Champions x2 or Items x2 (too lazy to search the links) but why not, after all? Also, unlike those previous ones, not EVERY value is multiplied, just one (or two, depending of the mastery in question)

This is designed to be some kind of featured game mode, and I agree it's totally not original, but no one thought of it, so... plus, I always thought (and still think) masteries don't have a really noticeable impact, so... well, maybe this version is too noticeable, but well... it's a featured game mode concept, it's not meant to be anything near balanced! It's here to have "fun"! But I agree, several should be banned or something like that...

Note: Several descriptions were re-written to be clearer about their new values.

Tier 1
  • Double-Edged Sword mastery 2014.png Double-Edged Sword
    • Melee champions: You deal 20% increase damage from all sources, but take 10% increase damage from all sources.
    • Ranged champions: You deal and take 15% increased damage from all sources.
  • Fury mastery 2014.png Fury
    • Increases champion attack speed by 12.5 / 25 / 37.5 / 50%.
  • Sorcery mastery 2014.png Sorcery
    • Increases cooldown reduction by 12.5 / 25 / 37.5 / 40%.
  • Butcher mastery 2014.png Butcher
    • Basic attacks and single target abilities do 20 bonus damage to minions and monsters.
Tier 2
  • Expose Weakness mastery 2014.png Expose Weakness
    • Damaging an enemy with a spell increases allied champions' damage to that enemy by 10% for the next 3 seconds.
  • Brute Force mastery 2014.png Brute Force
    • Increases level 18 attack damage by 40 / 80 / 120 (2.22 / 4.44 / 6.67 per level)
  • Mental Force mastery 2014.png Mental Force
    • Increases ability power at level 18 by 60 / 110 / 160 (3.33 / 6.11 / 8.89 per level)
  • Feast mastery 2014.png Feast
    • Each time you kill a unit, you regain 30 health and 10 mana.
Tier 3
  • Spell Weaving mastery 2014.png Spell Weaving
    • Damaging an enemy champion with an auto-attack increases your spell damage by 10%, stacking up to 3 times (max 30% damage increase)
    • The effect lasts for 5 seconds.
  • Martial Mastery mastery 2014.png Martial Mastery
    • Increases attack damage by 40.
  • Arcane Mastery mastery 2014.png Arcane Mastery
    • Increases ability power by 60.
  • Executioner mastery 2014.png Executioner
    • Increases damage dealt to champions below 20 / 35 / 50% by 50%.
Tier 4
  • Blade Weaving mastery 2014.png Blade Weaving
    • Damaging an enemy champion with a spell increases your auto-attack damage by 10%, stacking up to 3 times (max 30% damage increase)
    • The effect lasts for 5 seconds.
  • Warlord mastery 2014.png Warlord
    • Increases bonus attack damage by 20 / 35 / 50%.
  • Archmage mastery 2014.png Archmage
    • Increases ability power by 20 / 35 / 50%.
  • Dangerous Game mastery 2014.png Dangerous Game
    • Champion kills and assists restore 50% of your missing health and mana.
Tier 5
  • Frenzy mastery 2014.png Frenzy
    • Critical strikes increase your attack speed by 50% for 3 seconds, stacking up to 3 times (max 150% attack speed increase)
  • Devastating Strikes mastery 2014.png Devastating Strikes
    • Ignores 20 / 40 / 60% of the target's armor and magic resistance.
  • Arcane Blade mastery 2014.png Arcane Blade
    • Autoattacks also deal bonus magic damage equal to 50% of your ability power.
Tier 6
  • Havoc mastery 2014.png Havoc
    • Increases damage by 30%.

Tier 1
  • Block mastery 2014.png Block
    • Reduces incoming damage from champions basic attacks by 10 / 20.
  • Recovery mastery 2014.png Recovery
    • Grants 10 / 20 health regeneration per 5 seconds.
  • Enchanted Armor mastery 2014.png Enchanted Armor
    • Increases your bonus armor and magic resistance by 25 / 50%.
  • Tough Skin mastery 2014.png Tough Skin
    • Reduces damage taken from monsters by 10 / 20.
Tier 2
  • Unyielding mastery 2014.png Unyielding
    • Reduces all incoming damage from champions by 20 on melee and 10 on ranged champions.
  • Veteran's Scars mastery 2014.png Veteran's Scars
    • Veteran's Scars grant 120 / 240 / 360 health.
  • Bladed Armor mastery 2014.png Bladed Armor
    • When enemy monsters attack you, they are inflicted with bleed for 2 seconds, dealing 10% of their current health as physical damage each second.
Tier 3
  • Oppression mastery 2014.png Oppression
    • Reduces damage taken from enemies with impaired movement or reduced attack speed by 30%.
  • Juggernaut mastery 2014.png Juggernaut
    • Increases maximum health by 30%.
  • Hardiness mastery 2014.png Hardiness
    • Grants 20 / 35 / 50 Armor.
  • Resistance mastery 2014.png Resistance
    • Grants 20 / 35 / 50 Magic Resistance.
Tier 4
  • Perseverance mastery 2014.png Perseverance
    • Every 5 seconds restores health equal to 3.5% / 6.75% / 10% of your missing health.
  • Swiftness mastery 2014.png Swiftness
    • Grants immunity to slow effects.
  • Reinforced Armor mastery 2014.png Reinforced Armor
    • Reduces the damage taken from critical strikes by 100%.
  • Evasive mastery 2014.png Evasive
    • Reduces the damage taken from area of effect magic damage by 40%.
Tier 5
  • Second Wind mastery 2014.png Second Wind
    • Increases all self-healing effects by 100% while below 25% health.
  • Legendary Guardian mastery 2014.png Legendary Guardian
    • Grants 10 / 20 / 30 / 40 armor and 5 / 10 / 15 / 20 magic resistance for each nearby visible enemy champion within 700 units.
  • Runic Blessing mastery 2014.png Runic Blessing
    • Start the game with a 500 health shield. This shield regenerates each time you respawn.
Tier 6
  • Tenacious mastery 2014.png Tenacious
    • Grants immunity to crowd control effects.

Tier 1
  • Phasewalker mastery 2014.png Phasewalker
    • Recall is now instant.
  • Fleet of Foot mastery 2014.png Fleet of Foot
    • Grants 5 / 10 / 15% movement speed.
  • Meditation mastery 2014.png Meditation
    • Grants 10 / 20 / 30 mana regeneration per 5 seconds.
  • Scout mastery 2014.png Scout
    • Increases the cast range of trinkets by 150%.
Tier 2
  • Summoner's Insight mastery 2014.png Summoner's Insight
    • Reduces the cool down of summoner spells by 40 / 70 / 100%.
  • Strength of Spirit mastery 2014.png Strength of Spirit
    • Gain 1 health regeneration per 5 seconds for every 30 maximum mana.
  • Alchemist mastery 2014.png Alchemist
    • Increases the duration of potions and elixirs by 100%.
Tier 3
  • Greed mastery 2014.png Greed
    • Grants 5 / 10 / 15 gold per 10 seconds.
  • Runic Affinity mastery 2014.png Runic Affinity
    • Increases shrine, relic, quest, and neutral monster buff durations by 200%.
  • Vampirism mastery 2014.png Vampirism
    • Grants 10 / 20 / 30% life steal and spell vamp.
  • Culinary Master mastery 2014.png Culinary Master
    • Your Health Potions are upgraded to Total Biscuits of Rejuvenation that restore an additional 200 health and 100 mana upon consuming.
Tier 4
  • Scavenger mastery 2014.png Scavenger
    • You gain 10 gold each time a nearby allied champion kills an enemy minion (1100 unit range)
  • Wealth mastery 2014.png Wealth
    • Increases starting gold by 400.
  • Expanded Mind mastery 2014.png Expanded Mind
    • Increases your maximum mana by 20 / 35 / 50%.
  • Inspiration mastery 2014.png Inspiration
    • Grants 5 / 10 experience for every second while near an allied champion with a higher level.
Tier 5
  • Bandit mastery 2014.png Bandit
    • Melee champions: Champion kills and assists grant 150 bonus gold.
    • Ranged champions: You gain 30 gold each time you attack an enemy champion. This effect can not trigger on the same champion more than once every 5 seconds.
  • Intelligence mastery 2014.png Intelligence
    • Grants 20 / 35 / 40% cooldown reduction and reduces the cooldown of activated items by 40 / 70 / 100%.
Tier 6
  • Wanderer mastery 2014.png Wanderer
    • Grants 50% movement speed while out of combat.

Do NOT hesitate for a single second to give your thoughts in the comments section right below.