I started yesterday the same post, I hope you can take a look:
http://boards.na.leagueoflegends.com/en/c/gameplay-balance/QEEfqntJ-garen-rework-main-garen-diamond
Nevermind, if it's too long, here it goes some ideas:
Q - Decisive Strike: Garen removes all slows on him and gains 35% movement speed for 1.5 / 2.25 / 3 / 3.75 / 4.5 seconds. His next melee attack within 4.5 seconds will deal X + Y% of the target maximum health (+Z% per attack damage) physical damage and silence his target for 2.5 seconds.
W - Courage: (Passive): Garen's Armor and Magic Resistance are permanently increased by 20% of bonus Armor and Magic Resistance.
(Active): Garen places a defensive shield on himself, decreasing all damage taken by 30% and reducing the duration of crowd control by 30% for 2 / 3 / 4 / 5 / 6 seconds. Additionally, it activates Perseverance during its duration, even if Garen suffers damage.
Edit: (Option1): Also, it gives nearby ally champions a defensive shield, decreasing all damage taken by X% and reducing the duration of crowd control by Y%.
Edit: (Option2): Also, it taunts nearby enemies and force them to focus Garen for X seconds.
E- Judgment: Garen rapidly spins his sword around his body for 3 seconds dealing X + Y% of the target maximum health (+Z% per attack damage) physical damage to nearby enemies every half second. Garen can deactivate Judgment early by activating the ability again after 0.5 seconds. Garen cannot use Decisive Strike or Demacian Justice while Judgment is in effect. Damage to minions is reduced by X%. Garen ignores unit collision while spinning, but takes a 20% movement speed penalty when passing through minions.
The Q and E changes are for him to be able to build bruiser or off-tank and been able to deal reasonable amounts of damage to anyone he can catch, note that the %max health based damage ensures that if Garen catches (for some reason) a squishy target, he won't explode in 2,5 seconds while a 4-tank-items-guy is on him.
The Optional changes on W would give him a "skillshot" to add more complexity to the champion, but at the same time, giving him a tool to help teammates in teamfights.