Garen just needs something to do after he blows his cooldowns
Like, if you look at other caster-oriented Juggernauts, they're all still threatening in a fight even after popping their cooldowns:
- Darius has Noxian Might for huge AD steroid added onto every auto, and his bleed, obviously
- Illaoi's tentacles all swing after she uses spells
- Urgot can still apply passive with rightclicks
- Mordekaiser's ghost/ult DoT fills in the gaps between his CDs.
But not Garen. Unless Garen goes into an off-meta build like crit or lethality, all he's got to threaten you with is a modest amount of AD, usually from Cleaver only based on his most popular builds, and piss-awful attack speed. If he was stronger in an extended fight like a JUGGERNAUT should be, I don't think I would have any problems with him.
Here are 2 suggestions for this goal, both tied to his E: 1.) After landing X spins, gain bonus physical on-hit for the next Y autos (does not apply to Q) equal to some % of total AD. This gives him a stronger filling between his cooldowns and an incentive to stay in the fight.
2.) Landing X spins marks an opponent with the debuff for 3 seconds. Hitting the champion with any other attack consumes the debuff to deal a % of the enemy's max HP as bonus damage. Like how Alistar's E works. This would let Garen feel more comfortable beating up enemy frontlines and other fighters, since of his biggest issues right now is being a fighter who can't fight.
Both of these changes can solve a big gap in Garen's gameplay without the pain of a rework and without creating an apparently unwanted skill gap between his players across their ranks.