I'm not sure if the MsgPlus! extension could do this even if the developers wanted to haha, but here goes:
I sometimes find that I want to log into MSN to send a file to a particular contact, but I don't want to start conversations with everyone else at the same time, especially if I'm in a rush or I'm very busy (or I just don't want to be bothered). Appearing offline allows you to chat with specific people while remaining hidden, but you cannot send files or play games in this mode. Perhaps the ability to select a user when in "Offline" mode and have an option like "Appear Online to This Contact" would solve the problem(s). I think either a right-click option on the contact's name, or a button in the conversation window would be quite handy.
Just a thought.
Thanks for reading