With the new Shadow Isles Lore showing many champions before becoming part of the isles I think a Pre-Shadow Isles skin set is only right. I think it be awesome to get a visual aspect for some Shadow Isles champs before they became what they are now.
I'd love to see something like that. Kinda like the prevoid kassadin skin. I'd also like to see an expansion on the underworld skinline but I guess that's more of a Halloween thingy.
I feel like some might not really work though
For example, 
can't really work without her spider concept (unless she uses like spider robots or something)
has her rework so I'll save judgement on her for now
could be a knight in shining armor on his horse. W might be weird to implement, but his R could just be fellow knights charging with him
could shoot arrows with "strings" attached to them, though the strings have to disappear after the attack lands, otherwise it'll clutter up the vision space. W could be a robot as well.
might not work, seeing as he was just a researcher prior to his (un)death. He can follow the Elise route and use technology, but idk.
Since he was a spirit prior to his merging with the giant oak, maybe some sort of ghost maokai skin?
The mystery surrounding his origin is kinda part of his character, so it wouldn't work for him
Souls collecting might be weird, but otherwise this one could work well
Same as Eve, will have to wait until rework to really give an opinion
Blood Lord is a forgotten skin line that can pay riot some good bucks.
Potential Candidates:
