My suggestion is two things but they're related.

Bat Guano:

What is it? Bat Guano is bat poop. Guano also means bird or seal poop but those are different.

Where is it found? I think bat guano should spawn in caves as blocks and carpets and also be obtained from bats to give them some use.

Is it renewable? The blocks would be kind of like gilded blackstone with a chance to drop a random amount of bat guano items also I think they should be uncraftable. That would not be renewable. Another way to obtain bat guano could be from the bats laying the item like chickens lay an egg. I think ideally they'd only poop if players are nearby and maybe they should poop straight into jars and not poop otherwise. Guano would not have many uses so I don't think players should encounter it frequently apart from being decoration for caves. Also guano shouldn't appear when people don't want it that just makes bats more of a nuisance

Uses? Bat guano could maybe be used as a fertilizer like bonemeal as well as a main ingredient in Saltpeter

Saltpeter:

What is it? Saltpeter is Potassium Nitrate

Where is it found? Saltpeter should be crafted from ash, bat guano, and water (cocoa beans are guano and sugar is saltpeter) The recipe would be shapeless

Note: The water bucket doesn't need to be used up for this recipe.

Is it renewable? Yes, but not easily and that's a good thing based on its crafting uses.

Uses? While it is used as a fertilizer also in real life that would be lame and bad to do especially if bat guano is also going to be a fertilizer. Therefore I think it's more fun to imagine saltpeter being used only in excess which in real life kills plants. Saltpeter could be used to kill grass, resulting in a dirt which cannot be taken over by mycelium or grass. This would be a way to have true dirt next to grass. Saltpeter could also be used to stop plant growth similar to how Quark uses poisonous potatoes to stop animal growth. That would be good for decoration. Finally, saltpeter would be used to craft gunpowder.

The recipe is 6 saltpeter, 2 charcoal, and 1 blaze powder (OreDictionary sulfur should also be used). This is very close to the 75% saltpeter, 15% charcoal dust, 10% sulfur dust recipe of actual gunpowder. The recipe should also be shapeless

I forgot to mention another function of saltpeter and that would be to increase cooper's oxidization level. Perhaps instead of using the saltpeter item on the copper you can place a saltpeter block (which has the effects of the item in an area of affect) and that makes oxidization faster. Irl Saltpeter is an oxidizer.

That's a nice idea. Could be a reliable way to get gunpowder in peaceful, except for the blaze powder part