What they should do is make garen more specialized and able to contribute in a more meaningful manner. Already he is the tankiest juggernaut with the worst in-fight sustain, and he sort of acts as an execute assassin type juggernaut. Whereas darius is more offensive but takes longer to get ramped up, with less durability but more potential in-fight sustain. While I think darius should have a few changes that allow him to decrease the damage and offer a slight AS or MS debuff of enemies as he stacks them (in exchange for less AD bonus on passive), garen on the other hand, could be given some changes as well that might fit his identity.
- One idea could be to give him bonus damage when he is CC'd, like after being slowed, make his next AA or spell 25% stronger (not including ult, that would quickly become ridiculous), and after stun or knockup, the next AA or spell would be 50% stronger. He would have a 1 second cooldown on the affect. This route would provide him more damage, which you said you don't want, but which would fit his theme of shrugging off cc and ignoring pain. This would also incentivize opponents to kite properly, because if they don't kite properly they could end up taking even more damage than before.
-Heavy Armor: Garen gains .25 magic resist for every point in armor that he purchases (since his armor has petricite or whatever), and for every 100 total armor and/or 100 total MR, he gains 10% tenacity. This would definitely increase his survivability, but wouldn't be a great help against threats like tryn except for the fact that garen could build more armor with less attention to buying MR. If he simply bought randuins and ninja tabi, he would gain 25 MR and, combined with base stats, would gains approximately 20-30% tenacity.
-Q bonus - crushing weight. For every 500 HP and/or 50 armor garen has more than an opponent, he deals 8-10% more damage. This would. This would really reinforce his burstier nature against squishies when he's trying to target and pseudo assassinate them. He would still have the issue of GETTING to the backline or keeping up with squishier bruisers, as he currently does.
-Passive bonus: Hearty Warrior - Garen gains 2 AD for every 100 bonus hp he has, and gains 75 hp for every bonus 50 armor. In a likely scenario, he would gain approximately 150-225 hp, and about 40-50 AD
Just some thoughts. In order to gain some of these benefits he might have to lose the villain mechanic, but like you and many other garen mains have stated before, the villain mechanic isn't greatly appreciated by garen mains.
Other ideas would be to give him some form of skill shot or other type action.
One idea would be to give garen shields surrounding one quarter of his body each, after he uses ult. They'd last about 6 seconds and would absorb damage taken from the portion of the body they are on scaling something like: 100-200-400 (Increasing with each rank in R). So garen, if he effectively maneuvered could at lvl 16 reduce damage by up to 1600, but in most scenarios would only block about 400-800 damage late game. Still a significant boon though, that would offer the chance for some skill expression and allow him to do something with his ult other than nuke somebody. It would be similar in use to pantheon's new W, but with a finite amount of damage reduction, and would be sort of the opposite of fiora's vital spots concept.
early. this is what really negates alot of tryns damage
is alot better vs tryn then 
