The best option in my opinion. Maybe change it to allow all aspect ratios or maybe a toggle for limiting from 32:9, 21:9, 16:9, 4:3. Should be the easiest thing for Blizzard to enable, as the game is already rendering underneath the black bars.
This would let PVP players with 16:9 monitors disable the advantage in their competitive games. Such a button would also let ultrawide users battle each other in pvp without black bars.
Could be alot of fun too, imagine how many 4:3 barbarians it needs to take down one 32:9 sorceress.
It would fracture the player base as little as the “item spacing” button does. The one that lets controller players enjoy the game. Most people are inclusive and won’t care what type of controller or gaming monitors others are using.
If you want to play games that are restricted it should be entirely up to you and should not stop other everyone else from enjoying the game.
Having this toggle button is a good thing for the pvp community too, as it is more future proof and at least some will eventually want to pvp with wider screens.