The Meaning of Life, the Universe, and Everything.
The first mod,....
Ghoul mask: You remember the ghoul mask from Fallout?. Why not make it so that when you wear the head of a mob they won't attack you. And optionally you can even make it act like the mask from fallout so that if you get too close to the mob there's a chance that they'll see through your disguise and attack you anyways.
Beneficial/toxic liquids pool world gen: I've seen a lot of tutorial on how to create the illusion of fog using colored stained glass. But this gave me the idea for a world gen where pools of liquid that are brightly colors spawn. Green, Blue, Purple, Red. Whatever color you want. And these colorful pools of liquid have beneficial or even toxic effects. Like the green could be poisonous, the purple can be corrosive, the Red could be regenerative, and the blue could refill your hunger bar. Some of these pools can either glow, emit a fog of the corresponding colors or even do both. Just imagine adventuring through the world and come upon a wasteland biome with these poisonous green pools the spew forth a miasmic green fog that wilts plant life and saps animals of their life. Or you come upon a magical forest that has these glowing blue pools or nourishing energy that spawns new life around it. Pretty amazing find wouldn't you think?.