I thought it'd be fun to share my thoughts on what I think Riot's going to do with the various skins the spooky scarecrow has when the VGU hits later this year. He has quite a few, so I'll just get right into it.
Union Jack: This is probably the hardest one of the entire bunch. It's a weird skin that doesn't really fit into modern LoL, so I'd assume massive changes to it but... Twitch has a similar skin that wasn't heavily changed when he got a visual update. Plus removing a nod to Britain likely wouldn't be a good decision. So... They might spruce it up, try to tie it with a winter sport more directly. But I think the Union Jack will remain in some form.
Spectral: Thankfully we have an example in game of how this skin may go. Spooky Gangplank. It's a very similar skin, and we know what it looks like post Gangplank's rework. I imagine Fiddle's will be taken in a very similar direction to that one.
Bandito: Probably make it more similar to the modern High Noon skins. Make it look much more demonic and with a darker color scheme. Might be renamed to High Noon Fiddlesticks as well.
Pumpkinhead: This one currently is... Very plain and uninteresting. But it could be a love-letter to Halloween. Some references to candy, an orange and black color scheme maybe? I don't have super high hopes since it's a low-price skin and probably won't get unique visual effects, but there's still a lot of potential.
Fiddle Me Timbers: I... Don't know what to do with this. It's a silly skin and I think it should stay silly. But Riot might try to make it a more serious Bilgewater skin like the Cursed Tides event skins, or make it a Dark Waters skin. Possibly could get renamed, depending on what direction they take it. But I hope the old name is kept.
Surprise Party: The big ticket item. Fiddlesticks' legendary that currently... Isn't very legendary. All it has right now is some different effects and a fun noise when he ults. Fortunately it seems Riot's giving it a total overhaul, and bringing it up to the standard of other legendary skins, even if Surprise Party doesn't share the same price point. Fun potential for a silly legendary.
Risen, Praetorian, and Dark Candy: These are all lumped together because I don't think they'll be getting major updates. Risen and Praetorian are both newer skins, and already up to current standards. Dark Candy has aged really well and the thematic is fantastic. I think these skins will get updated in the same way Zombie Slayer and Baker Pantheon were. Given details from the new base champ design, but keeping the same theme and edited splash art.