It would be ironic indeed if this thread has been done before...
Let me tell you a story of a boy sitting in study hall. He is bored, having completed his classwork, and starts to think about what he often does when his mind wanders: Video Games.
He starts thinking about a concept for a game he's began putting together in his head the night before.
The plot was a classic version of 'The Magic Came Back', following the story of a normal college student(or something along those lines) in a big city who's just trying to find his place in the world, when, while walking home from college, he is mugged. After being shot by one of the muggers, a bolt of energy shoots from his hand. he makes short work of the others, and starts using his powers to increase his position in the town. He catches the news one night... "humans around the world are starting to exhibit fantastic powers that science can't explain!"
You can see where the plot goes from there. Villain is introduced, Villan X's The Protagonist's Y, Protagonist goes after X while trying to find out about where his powers came from.
The Gameplay would be an Open-world game, instead of using cars, you'd traverse the rooftops for the first half of the game, maybe getting some sort of flying object halfway around the middle.
Combat would be basic open-world game combat, using a free-aim crosshair like Saints Row's. As you got stronger over the game, you'd be able to do more things with your powers, both for combat and travel.
Now, the boy in the study hall owned an Xbox 360. He later told of his idea to his PS3 owning friend, and was almost immediately told that his idea sounded like a crappy version of INfamous.
The boy in the study hall was shocked someone else had thought of this, as he thought it was a unique concept.
It was hard to translate my exact feelings about the concept while typing this post, as i've forgotten some things about it. However, I've mentally changed the 1 game i'd get with my PS3 this year from Uncharted 2 to INfamous, and i'm looking very much forward to playing it.
After that long, long intro, i'll post the TL
TL
Has this ever happened to you? It could be anything, something as big as my discovery, or maybe something as little as an idea for a forum thread.