There are 3 skins I'd love to see...

Red Shirt Jayce. No special animations or anything, just a red shirt from Star Trek.

King Mundo. Elvis hair, white sequin jumpsuit, throws microphones instead of cleavers.

General Lee Sin. Confederate flag bandana across his eyes, plays the car horn theme from dukes of hazard when he jumps to people.

Black Mage veigar - exact black mage costume colors from the old Final Fantasy games

I want an ice crystal Shyvana not just ice particles but MADE of ice cmon it's the classic Fire/Ice pairing!
Twin Bite: The claws of course are the fangs of her ice-dragon form
Burnout an constant blizzard but softer particles than Sejuani's like a mass snowstorm not swirling ice.
Fireball: hehe, of course its a snowball
Dragons Descent: Leap through a wall of ice enveloping herself in it becoming an ice-dragon
*though being made of ice it would more accurately be called a dragon-golem.

I know it's already been done with Nidalee, but...

French Maid Fiora.

Instabuy.

I want to see Fire-fly Kha-zix
Add glowing butt, recolor body to black or dark green, and enjoy

Maybe nightmare Kha-zix
Shadowy wings, dark-blood red spikes, starts out a shiny black and evolving ult makes him a dull dark red. Glowing eyes. The whole nightmare idea, making Kha-zix look almost like nocturne.

Seems that Fire and Ice skins are pretty easy to think of and create

Shenderman.
He teleports to you, he forces you to look at him and he kills you...
Sound familiar?
As a bonus, attack animations aren't too bad because his swords can be represented by the long arms.