Hello,
I have a simple (I think) suggestion. I have seen in chat, and have done it myself,  that players get so involved with what they are doing that they miss other players trying to contact them.  My suggestion is

 /call [playername]

command that sets off distinct sound (enderman screech, maybe?) or flickers the screen on the players side (could be players choice). The sound effect could also be tied to the first private message on player sends to another. Of course, there are players who would see this as an invasion of privacy, so each player could turn receiving calls off, returning an AFK or "Player has calls off" message.

   Just a simple(?) suggestion. I am sure there are a lot of details that I haven't considered.

Craftymatt13

I love this idea, but it would need a lockout timer so people can't do it just to annoy you. Or just tie it into /ignore maybe