The actual content of a viral video can be anything from a chimpanzee doing kung fu to a hilariously low-rent TV commercial to a really great guitar solo performed by some thirteen year old in his bedroom. With that in mind, go ahead and develop whatever idea you like. Whether you're marketing a product or service or you're making short movies for fun, just fill your video with whatever you find interesting because, here's the trick: If you find it interesting, someone else finds it interesting, too.