Maybe some television or camera? Like in Hammer (If you would implement source sdk features that would be kinda funny) where you have a camera entity with flags like how big big is the spectate angle is, etc. Then you have a special texture that is the screen.
Or like a bullet screen wall - 2 walls which simulate the shots so if you shoot on the first wall, the second wall will have a bullet flying out of it with the same angle and bullet type etc vice versa.
On this way, you could make a competitve teleportal. Because you can see the screen behind the teleporter and shoot it to hurt them.
That is my most missed feature