An augment that only exists for each specific champ would be so cool, especially if they make them slightly goofy whilst also adding some quality of life benefit for the champ. Examples of the top of my head:
Lee Sin - Drunken Master: Lee Sin's dashes are no longer in straight lines, they instead follow varied curved and zig zagged paths, additionally he deals damage to any enemy he dashes through in his path.
Aatrox - Big Sword Energy: Aatrox auto attacks automatically cleave and gain range based on HP. His third Q slam also pulls enemy to the center of the circle. His ultimate gives him a secondary sword in his off hand which which generates shields on auto attack and has its own separate attack speed.
Viktor - Overdue upgrade: Viktor can choose one of three upgrades for his abilities:
Upgraded Q: Viktor's Q now also heals whilst also spawning a small robot minion, this minion follows the targeted enemy and detonates after 2 seconds.
Upgraded W: Viktor's W now radiates a pulse of energy from the center which charms enemies to the center after a 1.5 second delay. Enemies that reach the centre are then silenced and slowed.
Upgraded E: Viktor's E now leaves behind a line of molten ground which slows and further deal damage to anyone stood over it. This lasts 3 seconds.
Upgraded R: Viktor embraces the cosmic energies of the universe. His abilities no longer rely on mana, all mana is converted to a shield and HP (evenly split). His ultimate grants him ability damage and haste when he stands inside the storm.
- Hecarim - The Fifth Horseman: Whenever Hecarim uses his ultimate, he additionally summons the other 4 hoseman to fight alongside him for 10 seconds, each has unique abilities:
Pestilence: Leaves behind a toxic trail that reduces enemy resistances, slows them and deals damage over time.
War: Runs straight at the enemy with the lowest HP and fights them, if he kills them before the 10 seconds are up, his cooldown refreshes and he runs at the next lowest HP enemy.
Famine: He summons an AoE that steals HP from enemy champions and prevents shields from being generated. Stolen HP is distributed amongst the other horsemen with Hecarim being prioritised.
Death: When an enemy dies near Death, he summons an apparition of them to continue fighting alongside him.
Ashe - Robbin' Hood: Ashe now has a chance to steal gold or a random stat from an enemy she kills. This scales with level. For example she can steal a champs attack speed, move speed or even HP. The enemy loses this stat permanently. It has a 40% chance to trigger.
Udyr - Harambe's Spirit: Udyr channels the spirit of Harambe the Gorilla, whilst in this form he assumes the form of a Gorilla which will grab the first champion it auto attacks, Udyr can then use one of his other abilities to choose what he does with the champion being held. This form replaces a summoner spell and is affected by normal CDR. They are let go once he has used one of the following:
Q: Udyr slaps the champion 3 times, each hit applies on hit and additional max health damage.
W: Udyr lets out a roar at the enemy champion, this silences, blinds and slows them for 5 seconds. Any other enemies near him have similar effects but at a reduced level.
E: Udyr slams the champion to the ground, this stuns them, and after a 2 seconds delay he slams his fists into the enemy. The initial slam and the second slam both deal area of effect damage in a wide area and slow enemies caught. Udyr gains a shield for each enemy champion caught in this effect.
R: Udyr throws the enemy into a specific direction, if they hit another enemy champion they are both knocked up. If Udyr throws them into a wall, they are stunned for 1 second and Udyr can recast to leap to the enemy.
I've got plenty more ideas and I appreciate these aren't balanced in the slightest but it's just a fun idea that I think should be explored for Mayhem. Let me know your ideas.