It would be nice if communities that are similar enough could “share” a comment thread, so you don’t end up with comments scattered over many different communities for the same link. The mods could toggle something in the settings and say “This other community is good and we’ll be OK sharing posts with them”. You also wouldn’t have to explicitly crosspost.

Some apps will collapse those into a single post, but not all of them, and not all the time. It would be nice if that were better.

It would be nice if there was a way to handle instance/user migrations. If an instance gets their domain name taken away, there’s no way AFAIK for the admin to say “Here’s our new location, with a verifiable signature”. Likewise there’s no way for a user AFAIK to move their account with a verifiable signature that the new one is still them. Ideally this could all happen automatically with signatures getting synced automatically and all that.

I’m sure it would be a lot of work and no idea if ActivityPub would get in the way, but it would give people a lot more assurance that they didn’t pick a server that will screw them over by going down.