Let's just start things off with lore.
How about instead of him being a "Harbinger" that does absolutely nothing that his title suggests, you remake Fiddlesticks into a folk spirit _(Because what else can he really be besides a spirit/demon at this point?)_ Maybe change his title from **The Harbinger of Doom,** to **Herald of The Fields,** or **Herald of the Harvest,** something along those lines.
Make it so he goes about serving the whims of the peasantry and farmers of the world, both protecting and enriching their crops through his own wicked ways. Are the farmers having trouble with bandits or do they just really want to kill some person they're jealous of? They summon up Fiddlesticks who proceeds to go out and murder all of them regardless of the reason. After killing his targets he could take their bodies to feed the crops while also taking their souls to transform them into his cursed crows.
Next up could we please update his terrible animations and that awful voice? His laugh animation is literal garbage, he just stands perfectly still while doing a throaty smoker laugh, and his auto attacks are terrible. Why not let him be more like his appearance in the **Twist of Fate** cinematic?
_(Of course he doesn't have to have the twirl, that would probably be incredibly hard to make work in game, but at least let him lash out with his scythe instead of the: "I poke in your general direction, here's a transparent blob." that he has now.)_
He looked and sounded so damn cool in this animation, but then you get into game and it's impossible not to be disappointed at what you see.
And finally you can move onto his abilities and really it's only **Drain** that needs anything done to it if a revamp like this went through. Simply follow along with the theme and change the name to **Harvest** and there you go, Fiddlesticks is now a thousand times better than before and his balance hasn't changed one bit.