I understand what you're getting at, Garen's simple. He is designed to be simple. He'll never have a high skill cap and you know what? I'm totally ok with that. He gets the job done as he is now.
He becomes an unkillable force in the mid to late game with the passive and active effects of his W. Perseverance combined with a spirit visage is unreal, add in a Warmog's and you're good to go. That last change they made to him gave him exactly what he needed. He's not there to lock down opponents, that's what you've got your threshes, leonas, etcs for. He's there to soak damage for your team, make clutch interrupts to channels (ex Kat ults) and execute squishy/high priority targets. You start giving him more CC and he's going to need to take a hit elsewhere.
Honestly, he does his job just fine. He isn't popular because he's binary, he's simple. Like I said, he's simple by design. I think it was said somewhere else in this thread. He doesn't make flashy plays. He runs up to you, spins and executes. Rinse and repeat. Nothing real flashy about that. (Even though that ultimate is gorgeous every time I see it.)
To me, the concept of using an execute to snare somebody is just plain silly. I'm not saying this to be rude, I just don't think it makes sense. You used your ultimate to snare an opponent so you could catch up to them. Well now you've got no ultimate for when the rest of their team shows up, so your damage output drops like a rock. You have little to no kill potential.
And I hear this argument all the time that other champions do his job better, who can regen health, tank, silence and execute better than Garen? There may be champions who do some of those things better, but not all in the same kit. I Garen-tee it.
I understand you wanting to add more to him to give him more options, but he really doesn't need more options. He's got all he needs. Demacian Justice, baby.