Personally, I think Winston's biggets flaw is that he doesn't have any good damage output or burst damage.
Rein can 1shot a lot of characters with his charge, and can do crazy damage to the enemy team with his Ult.
DVa has crazy damage at point blank range, and can 1HKO every character with her Ult. Context specific*
Roadhog obviously has his standard hook combo that can 1HKO many characters, and many more characters if a Zen (or any focus fire really) is involved.
Zarya becomes a DPS beast when she's above 70 energy, and her Ult is 1 half of a tag team combo that can lead to multiple kills.
Winstons regular gun is the weakest in the game, and has no potential of being any stronger from his own accord. His Ult is not about damage, but about disruption, which has it's benefits, but in a game where deaths are crazy important, it seems lackluster in comparison.
This is why the Tank meta is a thing. Because most Tanks have 1HKO abilities to compensate for their low base DPS/mobility.
Personally, I'd love to see something like this for Winston:
Move his current primary fire to his alt fire. Make his new Primary fire a single electrical connection that does 1.8 time the damage of one of his current streams, but can't jump to multiple targets. Make it also consume his battery twice as fast.
Edit: though to get rid of the tank meta, you need to make it so Tank's simply can't do the kind of damage a DPS class can. 1HKO abilities are fine, but they should have big penalties and/or cooldowns. As it is, most tanks can land kills easier and safer than most DPS classes can.