What I think:
-Change passive to an in combat sustain. Maybe health regen that drastically raises the more spins he gets on a single enemy? This would reward continuously hitting a given target-and if he's in the middle of 3 enemies his sustain would be damned good.
-I think Q is more or less fine as is. I'm ok with Garen being kitable so long as he wins the trades he manages to get in range for...the problem is, he doesn't always win these trades. There are far more mobile champions I've lost to-no running on their part, they weren't fed or anything. Doesn't make sense to me. Shouldn't the immobile juggernaut win out if both sides hold their ground?
-I think his W should be reworked to something like Warwicks E, or Galio. A press and hold style ability. Maybe make the % damage stack up to a cap (say, 50%?) starting at a lower number. Perhaps if he channeled as I'm suggesting, he could have a brief period where he gets 1/2 his damage reduction, but for every attack he takes during that time, he gets an additional .5 or 1 seconds added to the time he blocks damage? This would provide counter play on both sides. Garen wants to use this brief window to make it last longer, but without taking a huge nuke during that time since the damage reduction is less initially. He wants to time it right-while enemies want to either burst him down during this window, or avoid hitting him at all so he gets less shield time over all.
-As I said earlier I think Garen should be better at extended trades, which is usually vs other bruisers he'll have trouble (though late game ADC's have been a cause for pain at times as well-they're not even running and still cut through me faster than I cut through them). Maybe a ramp up damage effect? %HP based, to target tankier targets? Unlike the current armor shred, have it ramp up each tick, starting with the 4th consecutive kit. This would provide counter play to the enemy to attempt to reset it-if you decide to stay in the spin, you should be punished appropriately.
-I love how his R works, but it is over shadowed by a certain Noxian. But I don't think giving it a reset is the answer-their ults should be unique. Maybe give it permanently reduced CD on each executed villain, down to a cap? Or additional damage for each execution. I'm one of the few who actually likes the villain mechanic-though it's admittedly odd to put an ability that requires you to reach a particular target to make use of...on a champion who has trouble reaching any given target due to lack of gap closer and CC.