I would love to have a "whiteboard" in the game. Something you can interact with and actually draw on using the mouse with a limited colour palette.
Sometimes when I'm planning out belt balancing or routing I like to draw it out, but right now I'm using a pen and paper, or paint.net, to do it. Then I thought... hey, doing this in game would be neat. Plus in multiplayer you could all see the drawing and talk about in game.