Nuclear holocaust: that looked like something very likely to happen when I was a child... planing how to build my own bunker was one of my favorite pastimes then!
The christian apocalypse or well, any other future where paranormal things become normal like the earth being invaded by demons, ghosts, vampires, etc.
The human race transmutes into something different, like all our consciences being transferred to a computer. Or amebas (has anybody read Seedling Stars?).
Idiocracy.
We get to a point where machines do everything for us (work, art, etc.).
Our creator reveals to us and removes all the suffering from the world. Everybody is happy ever after.
Genetic engineering becomes a commodity. Everybody can become whatever he wants.