Personally I feel that there are only a few people to strong
Macree:
He counters flankers, but being able to reset fan the hammer lets him kill tanks. Give fan the hammer a cooldown so he can keep the ammo reset on the roll but not be able to burst down tanks. He can still burst down flankers as is his job.
Widowmaker:
Personally she's to strong, in the right hands you can't even harass her since she'll snipe flankers with ease (Have a friend who has two sensitivity settings he flips between to quickly switch to and headshot flankers). Her only real counters are standing behind reinhearts shield and her own skill level, which unfortunately the former can be ruined by a junkrat/pharah/etc.
My only change though would be to increase the amount of ammo her snipe shots consume to 10, and her reload time by .75 seconds. This would give her 3 shots but give clear moments you can go in on her where she can't turn and headshot you. Additionally you could center more of her ult's power to herself by remove the persistance during death, but giving her unlimited magazine during its duration while increasing the volume so everyone can hear her.
Junkrat:
Personally, he's just to suppressive on defense. Really I'm not sure whats a good change, possibly reducing the charge rate on tire and reducing the damage on his knockback mine (using it pointblank is absurd). Although he's not super strong in one quality, he has a metric fuckton of tools (AoE nuke, AoE suppression, Deathtrap, Instant knockback, high mobility, post-death present). You can play him almost any mission in any role and his only real counter is a good pharah or a good widowmaker. Also on temple of anubisa double junkrat on defense is possibly the most accurate definition of cancer in this game I've ever seen.
Sleeper OP: Genji
Not sure what to say here, He's incredibly strong in the right hands... I've only seen it once or twice but god damn is his outplay potential crazy.
Edit:
Wanted to add that although I don't personally find Mei OP, I think her icicle should have damage falloff as it feels wierd for such a strong short range character to have good ranged sniping.