Day 1 progress:
Spent most of the day/weekend building the movement and grapple mechanics for the player. I ended up rewriting the entire thing from scratch as the original Rigidbody based character controller I was using ended up with a lot of physics bugs that I didn't feel like spending hours ironing out, so the player now uses a Character Controller (I know, kill me). The basic mechanics for the grappling hook is that if it attaches to a surface, it will pull you in that direction until you hit a wall, or cancel it with a jump. Cancelling the grapple with your jump maintains the velocity you gain from the grapple, and gives you an extra hop in the air which lets you get some crazy distance if used right. The video below shows a pretty standard way of reaching a higher platform using a grapple jump. The little white circle thing is the current "art" for the grapple, it disappears from the platform it's lodged in when the grapple jump is triggered (which is admittedly hard to see).
https://gfycat.com/LiveYellowishBeardeddragon
The next step:
The current mechanics are basically most of what the gameplay will involve, the next step is figuring out what the use of the mechanics are, and building assets to suit the decision. I'm debating between strictly linear-ish puzzle platformer levels with well defined goals, or a couple of large open areas that the player can explore at their own pace, with little challenges placed around and possibly some larger overarching goal. Smaller levels would probably require the overall speed and craziness of the movement to be dialed down while a more open area would let me design around the currently huge amount of speed that is possible through multiple grapple jumps. I'm erring on the side of the latter.
Oh and now I have to learn how to use Blender from scratch.