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.