An arrowhead with a grappling hook on it
This would shoot out of a bow or some projectile shooting device. When this hook hits a wall it sticks and then you might be able to Zip Line or something. Or when climbing is introduced (which is probably pretty far off) you could climb with it. Just a cool way to get around. Of course, you would also need to craft rope or something which would be a new challenge because of rope physics.
Mentioned this in my idea.
Being able to string together your own Zip Lines would be awesome, especially in the upcoming Forest update.