Underground mushroom/mildew

Mining is pretty dull so I suggest that you add some things like this.

There would be relatively rare veins of mushroom/mold blocks underground and there would always be several veins in a certain area. When you dig a block of mold it gives you a poison effect. Possible right way to get rid of mildew would be to burn them down but it would also be fun if it couldn't be removed any other ways than getting harm or using explosives.

An exciting feature with mildew is that when you dig a tunnel and don't notice the mold in its side it will resuscitate and start slowly blocking the tunnel. Imagine how annoying it would be to see these poisonous blocks in the end of your way. "Bah, now I must go round them and ruin my perfect tunnel system".

One else possible but a little lifeless feature is that mold blocks suck ores from their surrounding area and drop them when mined.