its not so much the specific weapons as types of weapons, sure you can make a champion with a gun (jhin) or give them claws but unique weapons all behave differently (this is coming from someone who plays dungeons and dragons).
ok lets use the OP example fiddles uses a sickle for his in game model but in the cinematic he indeed has a scythe, this isn't a problem itself but it does show the difference between in game and cinematic weapons on champions.
so my suggestion is that if a champion were to be made using a scythe or whip, they would obviously have a much longer attack range than a normal melee champ but shorter than even the shortest ranged champ (urgot), thresh is a good example of this because back in S3 and S4 he could use both runnans and hydra despite having what appears to be a ranged attack. he is in fact the only champion to change his attack animation based on his targets distance from himself. with his standard whiplike attack at long range and a less enthusiastic chop at melee distance. a scythe in this instance would function in a sweeping motion and instead of affecting a single target would hit targets in a cone or half circle depending on if its a single or double bladed scythe (or a full circle if its a two headed scythe with a blade at each end) of course it would also very likely have its own challenges and requirements for whatever champion its attached to because of animations and abilities and whatnot
A champion is a exemplar of skill and power, hence why they make it to the league of legends, or to some the inner machinations of vel'koz's mind. so their weapons would be quite different from normal weapons of the standard types above and beyond even simple changes