Sorry if someone's suggested this before... but how about a replacement for the Engineer's teleporter which instead produces a barrier between the two devices, sort of like the Hard Light Bridge in Portal 2? You could place it vertically to make a wall, or horizontally to make a bridge. Maybe when it's in wall mode, it would instead damage players who pass through, so you can't just cut off large swaths of the map?

edit: or maybe instead of damaging players, it could block only projectiles, so it serves as a source of cover?