Garen's ult should just give a 8s buff in a 1200 range area if it kills. It fits perfectly with his theme and it would make him at least not a complete troll pick. Demacia is all about happy friendship, Garen dropping a sword on some one and yelling DEMACIA would get people pretty hyped up one would think. You dunk some one and instead of it being a killsteal to a character that can't even use the money to carry, it snowballs the fight in your favor.
Heals for 33% of the damage dealt over the duration, 10/15/20% movement speed, 10/20/30 Armor/MR, 8 second duration, wouldn't be very broken I don't think seeing as the ult sucks ass right now and using it on anything except some one that's going to escape is almost detrimental to your team. This way you can ult the first target with the knowledge that it's going to make your team much stronger for the fight.
But alas, they've likely already considered something similar and didn't like it.