Your design document is really interesting, I like how it's kind of stream of consciousness writing. I was able to follow your thought process very easily and think about how I would design the game as well. You could definitely have the player unlock different gear as they ascend the mountain, and have some reason they have to go back down. But maybe instead of having ladders to get to areas faster, the gear they've collected could be used to accelerate the way they move through levels they've already "completed." Maybe a grappling hook rope (I'm thinking of like the grappling hook in Wind Waker) could make crossing a chasm you had problems with be over quickly. Or maybe they get chalk and can climb across certain sections faster than before. Your design doc inspired brainstorming in me lol, I look forward to seeing what the game ends up as! Good Luck!