Online/Offline - on the users profile it has if there online or offline.
Message - A button on the users profile to be able to send them a message. Could have an option for who can send you messages(friends only, no-one, extra).
Offline
For the first one, it would be cool if this would happen, but it may be hard for the Scratch Team to implement.
For the second one, that's private messaging, which is never going to happen here just for transparency reasons. If you want to talk to another user, comment on one of their projects.
Offline