Pretty much just the ability to turn on a notification for when players join or leave an instance.
I know mods can do that already, but I would much prefer an official implementation as opposed to getting a mod that may get you banned.
Since you can't really hide that you are in the world anyways because you show up in the social menu (And it shows up in the logs too), I think that it would be overall a nice feature to know when friends are joining the instance or when they left. Some worlds also have that feature where there's a sound playing whenever someone joins the instance.
Would be a neat quality of life upgrade that shouldn't bother anyone for as long as you can turn off notifications.