So I'm toying with a new mechanic for a champion that started out as an idea to make harder minions (for a new game mode). Each mutation would visually upgrade the champion and many would be mutually exclusive. Please submit your ideas - the more the merrier (preferably with some indication of the visual upgrade and mutation influence).

Cell Division Transmutation: Upon scoring a champion kill or being killed, you gain a random mutation from a pool of possible mutations. While the resulting mutation is random, your chance to receive a particular mutation is influenced by actions that lead up to Transmutation being triggered.
  • Offensive mutations:
    • Third Arm - In response to overwhelming enemies, you grow a third arm that autonomously deals physical damage with its attacks (has considerably less attack speed than you).
    • Sharp Claws - In response to heavily armored foes, your grow highly sharpened claws that (armor penetration).
    • Big Arm - In response to a preference to autoattacking, one of your arms grows significantly. Every other attack deals increased damage.
    • Adaptive Camouflage - In response to spending a lot of time in the brush, you gain stealth for 1.5 seconds upon exiting a brush.
  • Defensive mutations (damage):
    • Corrosive Blood - In response to overwhelming enemies, your blood gains corrosive properties. Whenever you take damage from a champion, you leave behind a trail of blood that deals damage over time to enemies who stand in the trail.
    • Reflective Carapace - In response to magic damage, your skin takes on a reflective sheen that reduces magic damage.
    • Hardened Carapace - In response to physical damage, your skin hardens making you more resistent to physical damage.
    • Spiked Carapace - In response to autoattackers, your skin grows spikes that inflict damage to attackers.
    • Shed Skin - In response to damage over time, you skin gains rapid regenerative properties. You take reduced damage from persistent and applied damage over times.
  • Defensive mutations (crowd control):
    • Ink Jets - In response to autoattacker, you grow ink jets. Attackers are blinded for 1.5 seconds. This cannot happen trigger against the same target more than once per a duration.
    • Increased Body Mass - In response to spending a large portion of fights in the air, you rapidly increases your body mass. Knockup effects are less effective.
    • Patagium - In response to repeated knockups, you grow patagium flaps between your limbs. After being knocked up, you remove all slows, gain a burst of movement speed and ignore unit collision for a short time.
    • Third Leg - In response to repeated knockbacks, you grow extra leg. Knockback effects are less effective.
    • Tentacles - In response to repeated knockbacks, you grow an array of tentacles. Upon being knocked back, you will grab the source and pull them with you half the distance.
    • Enhanced Respiration - In response to repeated slows, your respiratory system is overhauled. Slowing effects are reduced.
    • Disposable Limbs - In response to repeated pulls, you gain the ability to discard your limbs. You will resist the displacement of the next Fling Fling, Rocket Grab Rocket Grab, Dredge Line Dredge Line, Moonfall Moonfall, Apprehend Apprehend or Death Sentence Death Sentence, as it will pull off a disposable limb (you still take damage). The limb grows back each time you visit the fountain.
    • Nictitating Membrane - In response to spending a large portion of fights blinded, you grow a fully functioning Nictitating membrane. You will resist one blinding effect every so many seconds.
  • Utility/Other mutations:
    • Prehensile Tail - In response to repeated ambushes, you grow a prehensile tail. When an enemy engaged you from behind, they are briefly rooted. This cannot happen trigger against the same target more than once per a duration.
    • Thrill of the Kill - In response to being killed beyond the grave, you gain a preventative mechanism. a On champion kill, you gain a brief period of invulnerability.
    • Spare Organs - In response to a highly unfavorable KDR, you grow a large number of replacement organs. Upon taking lethal damage, you regenerate 25% of your maximum health over 3 seconds. This can only be triggered once per spawn.
    • Feign Death - In response to repeated executions, you gain a feign death mechanism. If you would take lethal damage from above 30% of your maximum health, you resist the grave on 1 health and gain stealth for 1.5 seconds - leaving behind a clone that dies in your place.
    • Adoptive Muscle Memory - In response to repeated deaths to enemies higher leveled than you, you gain will a small percentage of experience earned by nearby enemy champions.
    • Rapid Cell Growth - In response to long down times between fights, you gain bonus health regeneration when out of combat.

An alternative mechanic would be:

Cell Division Protogenesis: When you first rank an ability, it will begin cycling through a pool of possible abilities (similar to Pick A Card). Activating the ability will lock in the selection, allowing the ability to be used as normal. Upon respawning, all abilities much be reselected.