As @Mono Pixel said, Game Jams are about having fun and learning, there is no money at stake, nobody is losing anything if your game isn't what you hoped it to be. Everything will be fine, just enjoy the process, try and have fun and you will learn so much along the way. Who knows, maybe part way through you think of a way to make the game awesome, just make sure you finish on time. Good luck with this! Edit: proof of concept is completely acceptable as well, a very rough prototype is fine to end the game jam with, just make sure whoever sees it somewhat knows what it's about