What if Garen's W was changed into a modified version of Galio's old Bulwark?
Garen's passive makes me think that the idea is for him to be able to shrug off attempts to chip him down, and Galio's Bulwark essentially had that exact effect.
Here's Bulwark, for reference:
W: Bulwark
Range: 800 Cost: 60 Mana Cooldown: 13
Active: Galio grants the target allied champion or himself bonus Armor and Magic Resistance for 4 seconds, healing himself for the duration every time the target takes damage (each subsequent healing tick is 20% weaker than the last).
Bonus Resistances: 30 / 45 / 60 / 75 / 90 Initial Heal: 25 / 40 / 55 / 70 / 85 (+ 30% AP) Maximum Heal: 100 / 160 / 220 / 280 / 340 (+ 120% AP)
Something like this, though sans the "target allied champion" part and with some other tweaking on the numbers because Garen's a manaless non-AP champion.
Honestly, the main reason I thought of this was because of how it would interact versus Darius's bleed, what with the fact that the two are supposed to be in opposition thematically. Plus the fact that Garen's current W is an exceedingly boring ability.