Instead of giving Darius a full reset on a max rank ultimate...

Why not give it a full reset on kill but it places a mark on a champion that Darius dunks that doesn't allow Darius to dunk that target for a duration 90% of his ultimate cooldown? Darius's main strength in the laning phase wasn't that he roflstomps every melee in the game (though that is important) but it was that he could snowball his team as much as possible before falling off late game. You don't need to do anything other than this and Darius will be in a good place.