I flew around like 15 mineshafts, I see one trend.
Mineshafts are smaller overall, main tunnel + few straight side tunnels and that's all
Vanilla ones are complex, plenty of turns like labyrinth

Mod looks good and is presentable on showcases, so many people download it, but its not much entertaining to play
I love idea of these shafts from surface, maybe add shafts without ladder or ladder partially missing to the way down to make it more "abandoned", but ensure that height which player will be falling down surprised by lack of ladders will take not more than 8-9 hearts (to surprise player, not to kill him, which wouldn't be fun, but falling "almost" to death is funny)

Secondly, I like concept of main tunnel, but with not many side tunnels is boring...
It would be better if there would be more side tunnels from main one, and branch tunnels from side tunnels, like in vanilla mineshafts. Straight tunnels aren't fun, vanilla does it very good. Plenty of variation in directions, height length.
It interactive to navigate through unrecognizable patterns, if sth is very predictable, it gets boring very fast.

I'm hoping you see what I have at mind, I think these are really good points.