Buckle up lads.
I don't know if this has been previously suggested, but a Definitely Naut Blitzcrank skin? I didn't do the art, but I've seen it floating around. I think it would be a funny halloween skin. Another could be
Annie with Tibbers dressed up as Naut, but that might be too on the nose as far as the reference goes.
Cinco de Mayo Azir? Where his soldiers are stylized Mexican skeletons with designs similar to the skulls that the holiday is famous for.
Maybe not enough stuff for it, but you could also do Easter Anivia or Easter Syndra. Annivia would literally turn into an Easter Egg.
By the same token you could do Shacco. Although an April Fools Shacco might be better, although I'm not sure how you'd really convey the spirit of April Fools visually. Maybe confetti and such, something similar to Surprise Party Fiddles. (speaking of that fiddly gent, why was his ult for that skin never animated so that he pops out of a cake upon initially arriving? Rito plis)
Groundhog's Day Rammus. Okay.
Are we limited to current holidays? Cause you could celebrate ancient beliefs in constellations with champions that are already nods to them (i.e.
Skarner > Scorpio,
Cassiopeia > Cassiopeia,
Hecarim > Chiron,) The skin could be stylized celestial bodies. You could also do
Shyvanna where she only becomes a constellation when she ults. (Draco)
Why hasn't Garen had a Halloween skin where he's dressed up as Hercules? (since I brought up them greeks)
You could do Achiles J4 to go with it.
Infant Mordekaiser. His mace could be a rattle, he would wear a diaper, and his ult instead of being his helmet could be a pacifier (Halloween). Or on the other hand you could do Mariachi Mordekaiser. Give him a sombrero, a pancho, a Maraca etc. Perhaps it's too stereotypical, but you could probably figure something out for Mexican Independence Day. (Not a holiday suggestion but why isn't Morde's helmet animation, the one floating over people's heads when he ults someone, changed to a rocker hand sign like the one he was making in his old art for the pentakill skin, when using the pentakill skin?)
This isn't a holiday suggestion but I can't help it. Arcade Yorick. His weapon could be the guitar controller from the guitar hero games. His ghouls can be references to the pac man ghosts. (Yes, I know he only has 3 ghouls and there's 4 ghosts) He can be wearing one of those old school virtual reality helmets from the 90's. His dance can be another DDR reference. It also works with the team comp. (Hec jungle, Sona MF bot, Veig mid. Yorick top) His ult can be made up of lines of code.
Pumpkin Carving Irelia?
Okay, I've been wanting to suggest this skin since season 2 but never had the arachnads to do it. Spider-Man Amumu.
His q he slings webbing, his tears are actually spiders running around, his tantrum is just him lashing out, maybe throwing some objects around with webbing, idk what you'd really do with that, but his ult would be him shooting webbing out everywhere and catching everyone. If there's one suggestion of mine that the almighty Rito see's, let it be this one, plis.
Cotton Candy Singed? 4th of July.
Naughty List Syndra. She's been a bad girl this year and got coals in her stocking, and now she's Downright Darius and throwing them at people in a hissy fit.
Talon dressed as a nun? (lol)
Kinda wish I had more, but that's all I can think of at the moment. Maybe I'll think of some more later haha.
(p.s. sorry if it was tl;dr)