Since you cannot msg to people who are offline, what if you could "mail" people so they can see the message when they get on?
It is not a bad idea, but since we already have a discord server, where you can add friends, have voice calls, chat with each other etc, I believe that this won't be necessary.
I personally have discord, but not everyone has a discord, also, with a lot of people it can be hard to send messages to people who are offline.
It is very easy to download, and it is extremely useful as we are very active there. You can also send private messages, so whenever your friends get back online they will see the messages.
I agree with iwin1302. If you friend them, you could see when they get on and you could just msg them then. But this is not a bad idea either.