Aside from some way to move water/lava, I think I’d like to see ropes. I would draw a picture in paint or something, but I’m terrible at it and a rope is a rope, right? I think they’d fill a functional niche between grapples and the presumably up-coming ladders (I’m sure I’ve seen concept artwork for them) without negating the usefulness of either.
For a simplistic approach, you would either tie them to a block or fire one end with the same kind of mechanisms that grapples use. Then the rope would just unfurl down (either from the underneath or side of the block it is attached to) until it reaches another block or reaches it’s maximum length. Then you could climb up and down them with the same keys to swim up / down.
A more advanced concept would be that a rope would need to be tied ‘around’ a block, and could them be pulled / draped somewhere manually. However, achieving something like that would be a LOT more work and while cool, would be entirely overkill.
I think it has a lot of scope to tie (terrible pun intended) into existing features, and provides opportunities to expand on the basic principle. Perhaps rope climbing speed could be increased by having a higher strength / Endurance stat. Maybe basic ropes could break when more than one person is climbing them at a time, but different / better materials could increase that number before breaking, or maybe the maximum length a rope can unfurl to.
Also, somewhat more flippantly: It’s something vanilla Minecraft doesn’t have - That’s always a plus, right?