For me, pool party vel'koz would be one of the coolest skins possible for him. Pool noodle arms, beach balls for eyes, a little floaty vest for his armor, and a watery interior for it to all float in, in place of his glow. I don't see it happening but there's hope. After all, rek'sai got the jetski skin!
What's your skin that you really wish would be made, but you're just stuck being hopeful on?
Maybe AstroBard?
I think I'm just being greedy though - I've got AstroNautilus, after all.
heart seeker lee sin because love is blind
Noxus Soraka
Legendary tier, basically Soraka in more of a Noxians noble outfit, but incredibly condescending voice lines while healing allies and referring to the as her minions
"Good help is so hard to find" "You may be useless, but youll be more useless dead"
Well, my fave skinline is heartseeker, and my fav champ is Xerath, and the odds of that skin ever happening are pretty much in the negatives. None of the champs I play ever get cute skins :(
Lore based skins in general.
Hm, there are some coming to my mind.
Bloodmoon Camille
I think she would work really well in that skin line. In my imagination, she is based on the ghost tale of the kuchisake-onna. White mask with a mouth that is slit wide open, her legs are each one half of a scissor. Chains with kunai at the ends for her hookshot. Her passive would be either a blood orb swirling around her (AD shield) or blue ghosts (AP shield).
Chemtech Sejuani
I would absolutely love a chemtech motorbike. Basically Sejuani as a Zaunite, chemtech driven motorbike instead of bristle. Her weapon is a flask with green goo on a chain. Passive lets the tubes on her bike glow green if it is active and the Zaun crest on top of her. Q is a wheelie, E covers the enemy in green goo instead of ice. Same with R, instead of an icestorm in the area there are green fumes.
Star Guardian Caitlyn & Vi
I want them to be together in a team of Star Guardians. They would be so great. Caitlyn is the intelligent, responsible class represent. Vi is the always brawling tomboy. They always hang out together and no one understands why, since they are so different. Maybe hinting even some romance here, it would be so god damn cute.
Boneyard Urgot
Halloween skin. Urgot is an unholy abomination built out of the bones of different creatures. His arm cannon is the skull of a dragon and it shoots blue ghost flames. For his ultimate, he is opening his ribcage and his rips start moving like teeth, tearing you to pieces.
And the most unrealistic one: 
FEMALE ZAC
Give me a sexy slime girl like Suu.
https://images-cdn.9gag.com/photo/a0KmRPd_460s.jpg
Girl Gragas and Little Orphan Annie
POOL. PARTY. GRAGAS.
Star Guardian
Battle Bunny Jhin. [sg-jinx]
More realistically, more Debonair skins. Debonair Jhin, Debonair Sylas, Debonair Illaoi, Debonair Xayah........
Echo Fox/C9 Nasus.
The dream part is that I'd have to actually be the one to get it because good luck convincing any proplayer to use him at Worlds.
Thunder goddess Kalista, where she throws lightning at someone and when she kills with her E it “Smite’s them”. Q is same etc etc
It would never happen, but I'd love a Plush Toy Aatrox skin. Squeaky noises on Q are necessary. Based loosely on my boy Plushtrox of course: https://66.media.tumblr.com/bd9afc005fe3a3d7b1ca43f3d2244574/tumblr_pk6feip05T1r6hcp5o1_1280.jpg
Battle/Secret Boss Mordekaiser
Put more champs in cute onesies and pajamas.
That is all.
Lady 
Reverse skins, any that can be done.
Braum - A ram carrying shield with Braums face on it (i've seen a concept for this once, it was amazing). Kindred - Wolf is main body, Lamb the floaty one. Azir - Soldier guy summoning pigeons. Fizz - A tiny shark that summons a giant yordle Gragas - A walking barrel that throws fat people Rumble - Tiny Robot riding a mechanical Yordle Kled - Well, you know.
Jax ultimate skin but he spoke spanish and used a baguette (literally not even kidding i had this in a dream)
Cosmic Reaver Pyke. I want it sooo bad.
Main design:
His hook/dagger would be a sickle or curved sword.
His base model would have white and gold sacrificial/cultist style robes similar to Cosmic Reaver Kassadin.
His mask would be the usual Cosmic Reaver mask but broken in half. His eye would release stars and nebulas.
Abilities:
Most abilities would be purple/blue with galaxy designs to them.
Q would just be the sickle being thrown or stabbing forward, nothing too special.
W would put Pyke into a galaxy or deep space setting. Indicator would be stars or comets flying around his targets.
E would leave stars or constellations in his path as he dashed.
Ultimate would be one large constellation, similar to the cross shape that Cosmic Reaver Kassadin leaves uses when recalling. When landing the ult, the burst from the ground opens up a temporary small galaxy.
Voice lines (basically just rewrites of his normal ones:)
"I was born where stars died, I died where stars were born."
"Dark matter flows through my veins."
"Madness lurks above."
"Went too deep, part of me is still up there... in the void..."
I woke up and have never cried harder.
Ice cream zac. His blobs would be chocolate syrup with sprinkles
Pool party Taliyah where she rides a surfboard., throws either crabs or coconuts with her q, and she surfs a tidal wave on her ult
I have my shirtless Jhin(Dark Cosmic), so in terms of that, I'm good.
Candy Land Massacre Jhin is my top choice for a "sillier" skin for him, though.
Also, Dragon Trainer/Caretaker Bard.
Nightbringer Swain and Dawnbringer Irelia ... and a kog'maw skin that literally made him a cannon on legs, nothing more. nothing less
Order of the Banana Janna. I want to expand our council.
Spidurgot. He shoots webs with Q, venom spit for passive and his ult Strangles a target with a web and on reactivation he eats them.
Preatorian Ivern
Nidalee skins/chromas (maybe for Leopard), not the traditional chromas but like chromas where the Leopard skin can be changed into different big cat species, such as tiger or jaguar.
Vandal Kled, with Kled and Skaarl wearing leather jackets and being badass, and Kled making "brooom" sounds and pretending Skaarl is a motorbike.
2 main ones I've hoped for but won't get, because the champs are unpopular.
Kitsune Kindred (Art made by JellyEnvy)
Skin looks absolutely amazing and I really wish it'd be implemented in one way or another. It also gave me major Okami vibes.
Battle Boss Vel'Koz (Art made by Amuyopo)
Considering this years Battle Boss ( and Arcade in my personal opinion) selection, I think it's needless to say that this would have trumped the real skins.
Oh I thought you meant a skin that literally was in a dream. Because the answer to that is I had a nightmare where I was on the rift and getting smoked by a Thunderlord Brand.
But if its just skins I wish was made, Zil is getting a skin later this year and it being Dunkmaster/Three Pointer Zil would be great, it has such amazing sound effect potential, and making a bm character even better at his job.
I also wish the chemtech skinline was used again in general since I love the Zaun aesthetic, but the only chemtech skin is Trynds. (Which I do love.)
Elementalist Annie. Grown up and wiser with deep and sometimes twisted voice lines. Tibbers no longer a bear, but an elemental. Ultimate skin.
My dreams are simple. Star Guardian Yuumi for maximum cuteness
