I think the only overpowered heroes at this point are Widowmaker, Mcree and Mei. I suggest:
Widowmaker
-Double widowmaker's charge time on her scope.
-Make her ult's audio cue more noticeable
-Cause her ult to cancel on death
-Add scope sway when zoomed and flinching from damage taken
Mcree
-Increase distance on Mcree's flashbang projectile
-Reduce radius of flashbang explosion
-Require target to be facing flashbang explosion to be stunned
-Reduce damage of Fan the Hammer
-Reduce ult charge retention from cancelling deadeye to 25% (down from 50%)
Mei
-Increase time to freeze solid by 1.5 seconds