Backdoor Protection/Base Turret Buffs/Comeback Mechanics
Backdoor protection needs to be added to the game immediately. Buildings should need minions nearby in order to be damaged, since otherwise you can wind up playing against a team that is really bad, but they can just teleport into your base and kill your nexus without any minions nearby for no reason. It's incredibly unfun to play against, and super unsportsmanlike to do. It doesn't fit the spirit of the game, and needs to be fixed.
In addition, base turrets as they are now are completely useless, especially late game. They don't do nearly enough damage to anything, and are basically just minor annoyances to champions. They need to be buffed heavily to help prevent split pushing and backdooring.
Also, as it stands currently, there are no comeback mechanics implemented in the game, meaning that if you have a team of champions that benefit heavily from late game play, but are up against a team of champions that stomp really heavily early game, there's no mechanics to enable your team to make a comeback and potentially win. This makes games unfun to play against if you end up losing, and creates toxicity among players on a team who feel that they can't possibly win a game and need to surrender, but are unable to. Homeguard is already a form of comeback mechanic, but it kicks in far too late. It should kick for a team once one or more of their outer turrets are destroyed, allowing for faster responses to base turret and inhibitor defense, as in many games teams are able to push down to inhibitors before the 20 minute mark.
League of Legends can be a really fun game under the right circumstances, but as it stands there isn't enough in the game to help out teams that are losing, enabling for more comebacks and better games for everyone involved.