Login as a remote user using OAuth 2.0 and ActivityPub client-server.
This would simplify interactions with remote servers as you could just login to the remote server as your account on your own server.
Requires full ActivityPub client-server support and some way of creating OAuth clients on remote servers. (Federated OAuth clients?) Also could be abused easily. (Only allow basic actions like following, unfollowing and commenting and redirect users to their own server for confirmation.)