Show potential contents on pipes (and ghosts).

It would be extremely useful to be able to see what might come out of pipes and conveyors when you're in planning mode. A production building shows potential outputs on its ports, and I think this should be carried through pipes and conveyors (both built and planning mode).
For example, connect a ghost pipe to a diesel output port, and the far end of that pipe would show the out arrow with the diesel icon. Connecting the same pipe network to multiple different inputs/outputs would show the multiple-product icons like buildings with many recipes do (possibly with some kind of alert icon, since pipes are single-product).
This would make building complicated pipe and conveyor networks much easier, especially when the runs are long.