I understand that some points I’m trying to discuss here may be based on my incomplete knowledge of the current state of development. Sorry for that in advance.
Mineral related stuff.
Minetest really gives players infinitely more space to inhabit than Minecraft, but this space must be filled with something, right? For me, digging deep underground and finding only iron, coal, and occasional MeSe was kinda disappointing. I believe Minetest requires a bigger variety of minerals, but not in terms of quantity, but of quality. Various metals are all good, but that’s not all one can acquire from a mine. Also, metals can be used for other purposes than making tools and armor.
1. Colored glass.
One might use various ores in order to give glass different hues. Just think of it – decorations made not of plant-colored wool, but real stained glass windows, that color the light coming through.
2. Sulfur and saltpeter.
I don’t know whether Minetest actually needs explosives, but making them should be, in my opinion, more straightforward. Most other uses of sulphur (At least the ones I’m aware of) lie in kinda advanced (for a guy digging with a pick) industry, but can be implemented in the future. Gunpowder can be used not only for industrial explosions, but for making fireworks.
3. Mineral fertilizers.
Self-explanatory, really.
4. Precious stones.
For these things to have practical value, one has to acquire some pretty advanced machinery, but such things rarely stop the true laser and diamond tools enthusiasts, do they? Making a primitive lens from a polished gem is, I believe, possible. If the high tech route doesn’t seem feasible, think of the possible magical applications.
5. Magnets.
Aside from making realistic compasses, these might be used for constructing various other machines. Also, iron tools turning their owner towards a big enough deposit sounds ike a nice touch.
A bigger variety of minerals and their uneven distribution should create a need for trade and interaction between players.
A good thing in my book.
Awaiting your input.