Wow that's a really tough call - asking you to come up with a totally original idea for a game.
Is he expecting everyone in the class to come up with a totally original idea?
Or is that not what he really said? I'm not a bad programmer but I'm completely useless at thinking up new ideas for stuff to do (otherwise I'd be in business on my own not digging someone else's ditch), let alone completely original ideas that nobody could have ever published on the net.
Perhaps you could ask the teacher what he expects and how you could prove that you didn't copy it from online if it turned out you hadn't actually come up with a completely original new idea.