You shouldn't buff Pantheon and for one simple reason (I don't want Riven 2.0)
He able to even out proc time Riven on Conquer proc timer and that alone say if he got buffed in term of damage, he going to be busted and enough say his Bonus Q with conquer proc make him heal like 25% of his hp with enough AD.
So please don't buff his damage , either give him another rework or buff his defensive and nerf his offensive.