after you set up the zip lines and get quite a few of them coloring them would be nice so you could color them to know what direction you're going up or down or you call them in your own little freeway

With what is currently available you could use trail markers at a zipline. Personally i either use the dandelion tuft image or the house image since most of my anchors are built next to a base.

Very good idea. Adding new icons would be great as well. Not only for the zip lines, but for more items as well. A web, acorn, etc.