One way walls. These have been mentioned before, but no real discussion has ever arisen. These would be very easy to code, and would make an insane amount of new creations possible. Please, just add it to the game already - thanks.
That would be amazing - it would make it so much easier to make reactors, etc.
You could already do this with some powered pipes. But I do support this idea as not everyone understands pipes fully.
Pipes make the element lose some of its properties and are destructable. A one way wall that you use like a FAN would be good. When you set the direction of the FAN you just draw a line and it sets the direction. One way wall can work like this.