Ok, so this is a big one, but it would be very fun. I think it could be arranged so that it does not mess up the flow of the game. SUPERPOWERS. They come from superpower orbs, which are found on the moon and mars in alien constructions. They are also craftable with extremely rare combinations of extremely rare materials. When you eat a superpower orb, you never know what superpower you will get. The superpower will last either 24 game hrs, or until you die. The orb is not reusable. Some superpowers are useful, some are weird and a few are dangerous. Superpowers the orb can give you include: superspeed, flight, burning (without damage), shapeshifting (become a mob of your choice, and your name id is hidden, and you do the same thing that the mob does- hop, fly, swim, be small, shoot fire, be nasty, etc), invincibility, size changing, shoot fire balls, breathe underwater, antigravity (stick to walls, ceilings), hydroplaning, invisibility, plague (make everyone, plants, and mobs sick around you), super-strength (no need to use any tools, use your hand), reciprocity (anything done to you is done to the one who did it), super bounce, short range teleportation (point and clickto teleport), long range of tool usage, walk through walls (though it still hurts you). Every superpower should have a interesting, strange, or detrimental side effect. Perhaps the side effect is more useful than the power (example- invisibility causes you to glow- light up your surroundings). Double up superpowers sometimes. Sometimes the length of time that they last varies (usually about 1 day, but sometimes more).
When you get a power, an info bar comes up explaining how to use it. It disappears after 2 minutes or if you click out of the dialogue, and you cannot get the info again.
Also, if there are any radioactive blocks, they should give superpowers rarely. Certain rare events should cause them.
Additionally, certain mobs should cause certain superpowers, such as a spider should sometimes give antigravity (make a new spider to do that, that is rare), Mummies and zombies should sometimes cause plague, eating certain things such as mushrooms, should usually hurt, but should sometimes cause things like invisibility.
Also, certain foods should extend the life of the superpower, though still, if you die, you lose it.
Not a superpower, but you should have an uncraftable extra life coin that if you have it in your inventory when you die, you respawn next to your bones with full health, but no armor. One coin is used every time you die.
Another Idea is to have more common items that will give you some of the weaker superpowers for 60 seconds. (e.g. health regrowth, invisibility, breathe underwater, etc.)
BTW, invisibility should also make you invisible to mobs.