[Champion Concept] Whip-user
TLDR at bottom. There's a lot of text outside of it, so be warned.
There's not any champion that uses a whip (Thresh doesn't count since he uses the thing at the end of his chain, making it more like a kursai-gama). This void could be filled in plenty of cool ways.
These are a few ideas for an adc without numbers or names attached (I'm not the best at naming things). This is assuming the whip is similar-looking to those used in Castlevania and stuff like that.
Passive: Auto attacks deal bonus damage near max range (due to the snap at the end, could be a visual/sound to go with this, such as a small explosion or the whip glowing or something with power when fully extended like that). Possibly increase the bonus damage somewhat when critting to make it relevant late-game without making it over-bearing early.
Q: A line poke skill that does more damage at the end, similar to his/her passive. Not the most inventive skill, but it would work as his main poke skill. Probably has longer range than his/her autos. Maybe give this a brief wind-up time like Jinx's Zap to give opponents something to do against it instead of just being easy harass.
W: Wraps the whip around the target, preventing you from doing anything else (maybe letting you move at reduced speed) but either stunning or rooting the target while channeled. Maybe slightly let you drag them around but slow your movespeed greatly, like a mini skarner ult (though that may be too strong). Possibly does a chunk of extra damage at the end based on how long it was channeled if you weren't interrupted. Whip may need to be magical and/or be able to extend for this skill to work lore-wise.
E: Whip in a wide, downward arc towards enemies' legs, slowing enemies and preventing dashes and/or flashes from being used briefly (similar to a root but without basic movement being stopped). This is the main skill to at least somewhat let this ADC kite, but not an overbearing amount since there's no mobility on him/her, and the enemy could dash/flash afterwards.
R: Whips extremely rapidly in a cone in front of him/her, dealing bonus damage at the end and less the closer the target is to him/her. I imagine this would be like and ultimate version of a combination of his/her Q with pantheon's E, except probably much longer range than Panth's E due to the damage being larger towards the end and less close up. Possibly slows the target if they are close to him/her so it's not wasted if they come close, but that may not be needed due to the E working well with this.
Lore stuff:
Could possibly be a Yordle who either invented a high-tech whip or stole/received it from Heimerdinger or something similar. There could maybe be a little ball or something at the end of the whip that creates explosions or something similar. That would help reinforce the extra damage towards the end of autos/Q/Ult. The W's extra damage at the end could be the burst, which would explain why it doesn't activate if interrupted and does more the longer it was channeled (since the device was charging the blast). The Yordle's reason for fighting could be almost anything. They could be physically weak, and the whip doing a lot of the work finally lets them fight. They could have lost family/friend/loved ones to the void/shadow isles/humans (although hating humans like that might make him/her too similar to Rumble), and either be out for revenge/want to protect them. While I know this may be too cliche, some other idea I haven't thought of could still work with the high-tech Yordle whip concept.
Another idea is that it could be a person who was also corrupted by the thing from Varus's village. That corruption place probably isn't gone after going into just one person, and it's power is ripe for anybody to take. Instead of a bow, this person has an arm turn into a whip, since maybe they were an explorer or beast trainer or something similar (I know the explorer with a whip might be too Indiana Jones, but still). They could have just wanted the power, been lied to by somebody/influenced by the corruption into taking it, or been driven to use it to survive by somebody/something hunting him. If somebody drove him/her to take it to survive, it could be Varus himself getting revenge on them if they were somebody who attacked his village. Would make a really cool rivalry, especially since they fulfill the same role but hate each other and possibly fight for opposite causes (Noxians attacked his village, so I assume Varus hates Noxians. Not sure if it's all Noxians, but I doubt he views the country favorably either way). The person could either be fully lost to the corruption and wants to corrupt more people, or still fights for Noxus. If fully corrupted (I assume their whole body would be covered in a similar way to Varus's arms/lower half), their ultimate could be multiple whips shooting out of their body instead of just from their arm, which would be really cool.
Or it could be any number of other backgrounds. A whip-user has a ton of potential.
TLDR: A whip user ADC with a kit about dealing more damage close to their maximum range. Lore-wise could be a high-tech Yordle whip, Varus's corruption with a whip instead of bow, or something else entirely.