Something Garen needs is a rework to his Q, W or R, or maybe even all of them.
The way Garen's E functions ingame is very solid and helps him CS, which isn't too oppressive or useless either, but all of his other tools are selfish, greedy and give him tunnel vision.
For a tank, even BRUISER, Garen lacks the general idea of a tank, somebody who can occupy the attention of 5 people. His kit is laid out to turn him into a live version of Rift Herald with only one objective, beating either on the ADC or on the villain. His Q is a nearly useless tool in a fair teamfight and his W, aside from giving him damage reduction, is absolutely useless. No damage reduction can fix that Garen doesn't even GET to participate in a teamfight at high elo. Everyone knows how limited and short-ranged Garen is and they use the knowledge to play ping pong with him. As the front line tank, all Garen does is take damage, not much more than that. He has no gap closers when flash is on cooldown and when he has flash available, he's most likely going to use it to make an all in engage on the next best person he can hit before he's mauled to death by true damage.
People like to compare Garen to Darius, and it's really not that wrong to do so, because it helps you understand how huge the gap between Garen's contribution and Darius' contribution is.
Darius has an effective gap closer, his E, that combined with his ultimate can put up to five people on the enemy team on a centered spot, perfect for his mage friends to fire all of their skillshots onto five people at once. His Q heals him equal to the amount of people hit by the edge of his Q, which is an annoying gimmick that helps him stay relevant in teamfights. There's nothing more annoying than being forced to watch your teammates feed health to Darius, and sometimes not even they can prevent it from happening because he smartly uses his tools to do so.
Then, Darius' ultimate. Garen has a barely guaranteed chance at executing somebody at a specific percentage of health, 'barely' guaranteed, while Darius has an ultimate that completely defies all defenses and can be recasted if he kills somebody. Basically, Darius has Garen's ultimate, except it works properly and deals true damage, whereas Garen's ultimate, unless he attacks the villain, deals percentage magic damage.
Garen's ultimate is unreliable, it fails to hit the spot and there is no clarity whether or not his ultimate is going to kill anyone. And even if it does, he is not rewarded for it in the first place.
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, all of these items that are frequently bought ingame deny Garen his entire kit. Whether it's their active effects, their health or their Magic Resistance, Garen will barely be able to kill anyone if they have these items at their dispense.
Mercurial removes Garen's silence and lets people that suffer under his vendetta use mobility skills/flash, and get movement speed that helps them escape from his E shred and ult range.
If you asked me right now what Garen needs that he doesn't have already, I'll tell you that Garen's biggest problem is his role in the meta. What he needs is a rework that defines what he is, a duelist, a defender, or an aggressive bruiser with short temper. Right now, he's supposed to be all three at once and is unreliably doing all three jobs.
Garen doesn't feel or play like a tank, he feels like a duelist. He doesn't contribute tankiness to a teamfight, he contributes it to his own, selfish hate agenda known as the villain system. He doesn't care about the survival of his teammates, he cares about the death of one and only one person on the enemy team and has 0 control over who it is. It's a system that doesn't help him in the least, it only slows him down as both a tank AND a duelist.
The villain system is supposed to reward Garen, but it only restricts him. He can't reliably tank for his team because he doesn't have the tools for it, and he can't reliably outduel anyone because his villain system is randomized. He has no place in the meta because his role is too divided into multiple jobs rather than focusing on one task.