I'm not sure about a grappling hook, but this game will definitely need some form of climbing. Without it, the world will become very small considering all the space taken up by inaccessible mountainsides. I love the thought of climbing up the mountains of Skyrim, looking for handholds, with a slight possibility of falling to my death if I grab a weak handhold, wondering if I should have taken a path instead of this risky "shortcut". Climbing to hidden dungeons and dragon lairs that cannot be reached by any other means would be awesome too . . .
All that, and the ability to climb buildings in towns would really be great for stealth characters.
At the risk of being tiresome, i'll repeat that climbing really needs to be in the game. Even if we get some walkable mountain paths, we'll still be missing out on the thrills of mountain climbing. It would be a watered down game set in a mountanous region.
It just might happen that everything else in the game turns out great, but i will still have a slight bad taste in my mouth if i'm not able to climb.
Besides, it opens up such a vast horizon of new gameplay possibilities!