Ping tool like in Portal 2 co op.
Something that when you shoot it on a surface, it creates a small little "look here" emitter and disappears after a few seconds.
I know, I know, theres already a Portal 2 suggestion thread, but nothing little like this is going to get made from being suggested in this thread. I know this is an easy mod to make, and it really doesn't have to be Portal 2 related at all. It's useful for any teamwork stuff. Team leaders could have it in TDM to lead a squad. It's useful for a lot more than Portal.
Maybe an option that will send a little "look here" message on the side of a player's screen if they're in the same team or squad or whatever as whoever sent the message.