My favorite mod, Smart Moving, hasn't been updated in quite some time, it's still stuck on version 1.8.
It's a mod that gave you quite a few more actions that fit in very well with the style of the game, especially the exploration aspect. It allowed you to crawl to fit through one block spaces, grab onto ledges 2 blocks or taller, and everything in between. Even some more fun aspects like diving, sliding, and wall jumping.
I've seen a lot of mods around that allow you to change your characters animations pretty easily, like Quark and More Player Models. Obviously animations and actual hit-boxes are two different things however, but hopefully it's at least a good starting point for someone?
additionally i feel that i should add that any visual glitches with armor can be entirely ignored, it may bug other people, very strange people who wear armor for looks instead of protection but i have no intent of contributing anything towards fixing that issue and the accompanying huge amount of work.
Looks pretty good and works well now, but the controls are bit wonky. I tend to drop off the edges of stuff because it's hard to keep yourself oriented in a steady position, and you automatically drop when the ledge you're hanging on becomes blocked by a solid wall. Maybe you could do something about that. Also, the climbing speed in general is a bit slow. Climbing up single blocks takes longer than just jumping up, and climbing two at a time takes longer than just mining away the block on top would. With some improvements to the movement speed this would be a fairly nice alternative to Le Parkour, though.