Have you realized how few supernatural mobs are friendly? (none of them)
Personally, I think it is about time that we add a mob that can become an ally, if the player takes the right precautions. Enter: The Knocker.
A Knocker is a fairy from Welsh mythology, that is said to lead miners to the motherlode if they are treated with kindness. It is an old minor's superstition to leave food for the knocker to appease these little leprechaun-like fellows. There are even a few legends of miners being led to safety after cave-ins by the mysterious sound of an unseen person knocking on a wall.
If a knocker were to be added in-game, then they don't even need a model that can be interacted with. All that they need is some creative coding.
For example: The knocker can take residence in certain cave biomes such as abandoned mineshafts, or newly added "whispering caverns".
If a player were to mine in one of these locations, and add a cake or other type of food on a special table, then come back,they would find it had disappeared. Continuing to do so would lead to positive results, and the player could follow audio cues to something cool like a dungeon, stronghold, or vein of diamonds. There could even be a unique particle effect accompanying the knock, which can help lead the player the right way.
If a player doesn't feed the knocker, but starts mining from it's cave anyway(pr leaves junk for it to eat), then it is likely to play pranks on them such as making a loud sound that draws mobs, or lead them to dangerous areas with it's knocks. It could possibly even cause random cave-ins if the player is especially rude to it.
Part of what drew me to Minecraft when I was a kid, is how mysterious and spooky it once was. This might bring back that magic.