Literally everything but simple flat stats could disrupt other content and even then whoever has the bigger numbers would be the same.
Bring back tier sets as cosmetics at least but the argument about them being bad for PvP or “Shoehorning” people into certain builds doesn’t hold much water.
The issue I start to see arise is the multiple gear sets one, where you have multiple sets filling your bag spaces just because you do M+ and raid. Which is why they changed sets to change with specs in the first place partly (I think).
No tier sets usually come with some type of power that will no doubt make them mandatory, forcing people who only M+ to raid in order to be successful. Tier sets bonus MUST only work in their corresponding raid. They are a mean with overcoming the challenges of the raid. If you are only a heroic raider then you will/won’t get better gear from mythic plus.
If you want tier set bonuses to exist this must be the trade off. And no to open world too.
Sets were fun and it also meant they could be tailored to each class. Shared sets are 1) boring because everyone looks the same 2) suck for some classes because the itemized stats budget is bad or mediocre for most classes.
I think they should bring back raid sets, dungeon sets, AND pvp sets. The bonuses can work in any environment, but the bonuses would be more suited for their corresponding content.
I do think an argument could be made to disable certain pve bonuses/trinkets in rated pvp, however.