Honestly, too many to count so I'll just go through and see who I feel I hate the most and we'll see what we get.
- Everyone knows how she is so I don't really think I need to explain much here. Can't really say what I would want to change since I lot has already happened to her ( I mean, didn't she lose stun on Ult and W stealth changed?). Maybe make her E only work at a certain range like Lee Sin Q so you can actually try to get away from her if possible? ( also no more Eing into fountain deaths)
Never have liked playing against her or trying to kill her twice before rebirth. At minimum maybe reduce stun duration on Q a bit?
Don't like how she can just hold onto that stun. Would rather she tries to build up to the stun and can hold it for a duration before it's lost. Maybe as some composition do something to ult, like ministun for compensation since she needs to hold onto stun for that gimmick.
Don't like getting hooked to immediately get knocked up. First off: pull back Q length buff. That was stupid. Maybe try to fix hit box for that afterwards?
Just melts everyone. Plays almost support now and is just a pain in the ass. Maybe reduce his % health damage or make some abilities slower, like Pillar since you typically try to use it on stunned enemies. Maybe make the 3 hit explosion go off slower.
From my experience of her, I just can't win. She does true damage, she's mobile, she has survivability to an extent, it's ridiculous. Reduce shield a bit, or lower % of true damage. I've just never liked this champ since her release.
Can just delete you and then recall to get health back or safety. Maybe reduce burst, lower W stun at early levels.
Constantly abuses on-hit items. Maybe reduce certain items to melee (a lot of people have suggested this and I agree with them)
Make it so her Q only refunds after hitting a champ than just anything. Could reduce true damage done on vitals. Maybe even lower time when vitals are available and up.
- Another mobile champ that just does lots of true damage and just feels overloaded. Would rather see her be shifted back to a shield breaker and no auto mechanic back. Lower heal on Q maybe, remove mark from ult, I really can't pin down what I would change the way she is now.
He just is a purple eggplant that hops on you and beats you to death. At most lower attack spd gained from passive a bit, maybe lower stun duration, or make his ult focused on building some defense if he wants those bonuses.
Pokes you from range, hops in, burst, knock away, rinse and repeat. Maybe lower his range a tiny bit or lower damage on Q projectile. Lower knockback distance.
Always face her now and again and is either top lane cancer or just a super annoying support to deal with. Kind of feel like cheating but would rather just see a rework of her since I hear a lot of champs just do the things she does but better. Only exception that she can do multiple things.
Don't face too often but can't really stand facing when I do. Probably scale polymorph so early ranks aren't super harsh but can scale to what they are now I guess?
Ult just isn't fun to play around (especially when he goes full AP). Would add some form of counterplay to maybe get a bit more reaction time to see it coming but still have it be the same just a bit. Like a quick delay that you can see but still need to act fast on.
He's just a mistake. Can regen out of combat or if he can get away when caught (which is easy for him), his stupid execute and bonus gold. Honestly, he either needs to just give all the gold to a teammate if they helped out and remove his second portion of cut if he wants to be a solo assassin. Maybe reduce health regen amount as well.
He. Literally. Just. One. Shots. You. And Riot is apparently fine with this. I would say that he needs to have his jump a bit slower if he's farther from his target (if that's not already a thing). Maybe try to open his window of burst so you don't just see his ult icon and then black and white screen.
I hate how she can just build so much CDR and damage right now. And Riot has buffed her a lot and haven't really reverted her buffs when she was strong. First, CDR limiting. In general, there could be less in the game. Second, maybe make it so she can only stack one of her passive and play into the whole 'weaving autos with spells' concept. Could also have it so she needs to deal damage to build up execution on her ult, either in damage or range even. I really hate this champ.
He's just not fun. Make the whole game about babysitting him so he can't split push and trying to get a lead or else he stops you and you can't fight back since he'll have his ult. Again, feels like cheating but he needs a rework. He's not fun to play against and he's not fun to play as. His ult is a gimmick "get out of death for a few seconds" card, he almost always has his E up since he has built in crit and builds it + CDR, he just needs to be reworked.
Never have liked her. She pops off too much nowadays. Make her ult a flat duration, make her deal magic or bonus physical instead of true, lower the true damage, remove/lower the Q CD bonus during ult.
WE ALL KNOW WHY WE HATE HIM!!!! Dashing around, windwalls, crits, stupid placement of R when near towers, etc. Some ideas could be that crit damage is further lowered, like 175% or even 150% (maybe too much). He's someone that is typically getting full crit with two items and build whatever he needs afterwards. Or his windwall could have scaling duration, reduced greatly at early levels to the duration he has now at max. It could maybe block a number of projectiles or certain amount of damage. He could have a dash limit on E with it getting removed at max rank, removal of shield, reduced AP after ult, just a lot of things to make him less annoying.
AND THE LIST OF CHAMPS THAT I ALSO HATE BUT COULDN'T THINK OF WHAT TO CHANGE ABOUT THEM:

(I'm starting to think I really just hate League nowadays)