[Champion Concept] Barbie (yes, the doll)
Barbie (Barbara Millicent Roberts) obviously wouldn't be usable as a name, so you'd have to go with some other name, but you could have a lot of fun with the design concept.
Passive: Everyone Loves Barbie! All champions on Barbie's team have their passives enhanced slightly. cooldowns are reduced by 5 seconds and buffs to stats are enhanced by 5%. Barbie also gains the passive of the characters she wears the clothing of, which temporarily replaces this passive.
Q: Mistress of Wardrobes. This is a skillshot that, when it hits, deals light damage and changes Barbie's outfit to match the target hit(probably just colors or such). Mistress of Wardrobes then copies the Q ability of that champion. (in certain rare circumstances, such as azir, it might be replaced with their w or e). Targets hit in this way are embarassed that Barbie looks so much better than them, that the cooldown on that ability is increased by 3 seconds for them. (lasts 20 seconds, cooldown 10 seconds starts after end of duration or on failed skillshot)
W: Wrapped in plastic, it's fantastic! Barbie heals herself and the nearest ally. If she and that ally are both in melee with an enemy, the enemy is demoralized, and has 20% reduced damage for 1 second. short cooldown.
E: I can do that! Barbie copies the skill last used against her (damaging her or debuffing her, healing her or buffing her) and has all numeric values of that skill increased by 50%. long cooldown.
R: Pink Cadillac Convertible! Barbie summons a magical pink convertible and honks for her allies to hop in. Any ally that hops in increases the damage by 10%. The convertible drives until it hits an enemy champion, running over and executing minions in the way. Then Barbie blames the crash on someone else, and Officers Vi and Caitlyn write her up another ticket.
I just thought it was funny that League has so many characters with all these different outfits and they haven't parodied Barbie yet.
Also admit it, you want the convertible ult on your team.
) don't have 0 cooldowns.