I think a grappling hook would be a good thing for minecraft. My reasons for thinking it would be good is because how many times do you find yourself trying to get up something and just can't? Or trying to climb a mountain and spending forever doing it? It would work like a fishing rod where you right click to throw it out and then right click again to pull yourself up. The crafting of it would be a bow, string, and arrow. The concerns I have for it is it failing when you are in the air and you fall to your death. Also another concern I have is that the arrow would fail to shoot far enough and it would hit the ground or something and pull you off of what you are on.