To be honest, reducing all the damage so that there is no more one shotting people unless really fed. Before, you had to actually farm and cs to do as much damage as you do now at level 4. This forced players to have to get good at the basic mechanics, like csing, positioning, and learning when to trade, and some of the intermediate mechanics, like wave control, animations, and learning how play against enemies, in order to separate yourself from an average player to a good player. Before, your gameplay was a more accurate predictor of how well the game would turn out. Now, it's all about who can one shot who more. It went from who got the first items to who has the better champ in champ select. It's not fun getting one shotted by someone I didn't deal with for 15 mins just because my team fed them 3 kills. The slower paced games were better and they were better for League. Despite how many people complain about it being too slow, it is overall better for the game. Think of it. When you watch anime, would it feel more suspensful and satisfying to have 1 or 2 episodes building up to an epic fight or would you rather see the characters going all out every episode of the series? Most people choose the build up. League is no different. Even with the season now, games are still getting dragged on for 30-40 mins in low elo and 20-30 mins in high elo. Except now, the game went from actually playing the game to pressing QWER and hoping you can get the kill before your opponent finishes pressing his QWER. There is no build up. It's straight up fighting level 1 throughout the game. ADCs have too much damage early on, Assassins deal wayy too much damage at all stages of the game assuming they are not behind, Support champs have way too much damage for them to be support champs and have no gold, Bruisers are dealing too much damage for them to be too tanky.
After that, they need to fix their runes. The resolve tree has no MR/Armor. It only has health and boneplating actually sucks. So now tanks are running around just as squishy as mid laners. There are no bruiser runes. Conqueror's was a poor attempt at recreating Fervor of Battle. Aery needs a nerf. It shouldn't be able to poke champs and give its teammates shields.
Then they need to cut down on the CC. Almost every champ in the game has some form of CC from slows to stuns. It's annoying as fuck to farm all game, get ahead, and then not be able to play because I'm getting CC'd for years. Decrease the duration of CC so we can actually play the game and remove CC from some champs kits. Jayce's knock back, Leona's ult, Talon's rake slow, etc.
Bring back Banner of Command.
Turrets are weak as hell. Anybody can dive anybody and be safe. If you build health, then turrets don't even exist. The damage needs to be increased so when people decide to dive you, there are a lot more consequences. Tanks can sit under turret all day long and the turrets won't deal any major damage. I say, increase the base damage and damage scaling. The Turret range needs to be increased some more so that long ranged champs (Teemo, Jayce, Caitlyn, Lux, etc) can actually get targetted by the turret when they decided to poke non-ranged champs under turret. You can't even farm under turret without getting poked out of lane and that's supposed to be your go to when you get countered. It makes the game unplayable from the jump.
There is no diversity in item selection.
Increase death timers early on not decrease them. If you make a bad play and die from it, you deserve to wait to spawn in after a good amount of time. Not 3 seconds then it's back to lane. This creates a safety for players and discourages them from avoiding dying. Now you make it as if dying early on is okay since you really don't suffer much.
This one is my personal opinion, but remove the jungle or at least remove the smite items. You can increase the jungle camp experience and make it easier to farm them as compensation. The jungle makes creates a problem where a problem should not be. The jungle removes all the hard efforts you put into your laning phase and nullifies them. You're outplaying your laner, you get ganked and now you're losing lane. You want to go in for a fight, their jungler comes in a turns the tables. For all of that, you might as well take it back to 2 top, 1 mid, and 2 bot. Don't get me started on smite items. You're winning, the enemy red smites you and now you're losing regardless of how fed you are. Either remove the smite items or allow everyone to build the smite items.
(and Champs Alike) cant sit in a lane forever
so that champions can't life steal off minions while tanking the tower. (Nasus and XinXao)
So that 
cant abuse Life steal items from possible future AP lifesteal items.
So poke comps have a place.
So assassins have more windows to actually go in.
So that 
is even more threatening.
To trim the power that adc's abuse.
So that Healing supports are less oppressive.
(lets say Mundo has 2025HP his heal cap is 4050HP) So enough to sustain him during a fight.
Now same goes for adcs, Jinx has 1800hp, her heal cap is 3600.
