Had a couple of thoughts:
With regards to gating progress, in a similar vein to what I mentioned about zinc and taking ideas from Blood N Bones, and as mentioned previously you could have a world with no food, and no plant life, where you must produce food artificially with MineChem - the way to arrange that would be to have all the MineChem stuff require a specific metal only findable in that world! Perhaps you could have it hubless as
@lenscas mentioned - devastated and dead, so you have to wander to find things. You could even start in a dark manner by having you, on arrival in the world, having to use a mining drill belonging to a corpse at the gate until you find more stuff to use
Also, with regards to a Skyblock, it's hard to find things that haven't already been done, but I had a thought - inspired by the first novel in the Death Gate cycle: a series of tiered discs, one after another, representing society - the oppressed workers at the bottom, teaching you their Smeltery techniques, moving up through the echelons until you reach the (literally) upper class, who hold (and jealously guard) the secrets to magic. In order to open the dimesnional gate, you must bring the discs back into alignment, bringing equality to all the levels (as they're meant to be) and breaking the ruling classes tyranny!
It's a bit more role-play than most, but could be an interesting change
this isn't meant to be me telling you what you should do, more to hopefully inspire ideas and thoughts!