Currently there are some botlane champs that are totally unviable:
Pyke: Pyke is just way too slow for anything to land. With a Pyke as support your playing a 1vs2 botlane (GL winning that), buff Pyke.
Lulu: argubly situational and can work if the other team has mostly meeles and there is someone on your team to protect that deals lot of dmg. But overall weak even then.
Rakan: suffers the same issues as Pyke. Super slow spells that are way too easy to dodge. Even if you are stunned you have an additional minute to dodge his spells.
Sona: high risk low reward kind of champ, requires massive skills to play her (one small misstep and she is done), but unlike Zyra/brand her damage is null.
Kalista: needs a buff of some sort.
Tham Kench: people think u trolling when you pick this champ.
Nautilus: just not viable, even if you land a hook u loose the trade.
And there are some champs that are just too good and need nerfs:
Vayne: most forgiving/scaling ADC in game, almost everyone bans her, winrate is good, pickrate too, should be enough to justify the nerf, let other ADCs shine too.
Sivir: her autoattack animation is just too smooth and Sivir is too good at both splitpushing/teamfighting and 1v1.
Draven: only need one bloodthirster and there is little you can do all of a sudden.