When it comes to design, I feel that Fiddlesticks is in a weird spot.
Many parts of his niches are already kinda filled....
You are right that it would make sense for him to be a demon ap diver with a scythe, however, the problem for me is that we already have a diver with a scythe and kinda demonic features:
(Rhaast, to be more specific)
Another niche are his crows. They are a major theme in his design, having crows following him around the map would make sense.
But well....

Swain now has ravens everywhere which rip out your life essence and they appear everywhere he goes.
Because of that, I would take Fiddlesticks further away from his original design.
Get rid of his crows, including his ultimate.
I think they should focus him more around his ability to suck out your life.
And with that being said, I imagine new Fiddlesticks to be a champion from the Shadow Isles.
He was once a cruel man, but the ruination destroyed his body completely. Just his ghost remained and this ghost was looking for a new body as soon as possible.
So he just possessed the first thing in his reach.... a scarecrow.
He is draining the life force of people to gain power, hoping that he can one day restore an own body again.
Weapon choice could be a scythe, I guess. But I wouldn't rule out to give him another weapon. Maybe (for example) he just enhanced his scarecrow arms with some metal claws. Something that makes him look more scary.