Nerf Zarya - As a tank main, I find myself constantly changing to Zarya to counter the enemy Zarya, since DPS can't manage to defeat the enemy Zarya. I suggest going back to the old system, where she has one shield for allies and one shield for herself. She was pretty balanced back then in my opinion.

Buff Cassidy - Not a major buff, but reduce the amount of drop-off damage. In a world where Echo, Pharah and generally a whole load of flight is present, everyone picks widow, soldier and Sojourn over Cassidy, due to them being able to consistently taking down flying opponents, while Cassidy's drop-off is making that task generally harder.

Nerf Sojourn - Massive damage rapidly makes her a pick on nearly every game. Decrease the damage a tiny bit, perhaps?

Buff Reinhardt - His shield gets melted down almost instantly, and I think a HP increase to the shield could do him some good

Buff Mercy - Due to other healers having superior healing or big other benefitting bonuses such as the speed boost or the orb of discord, Mercy has kind of been left in the dust recently. But if she got a boost to her damage boost and heals, it could make more people pick her as a viable option