I think the entire concept of "robots in disguise" was vastly underused and has potential for a lot of different story elements that were never used. Like a Decepticon shooting at a yellow camaro only to find out that the Camaro wasn't really Bumblebee with the actual Bumblebee behind him. That would add a sense of suspense to the movie instead of blindly shooting at everything that moves. It would also greatly improve the use of espionage and spying as no one would ever know what was or wasn't a TF. Even just using a scene reminiscent of Knight Rider where the driver of the car talks to the TF via the dashboard would add quite a lot to the story and expand greatly on characterization while being extremely cheap to pull off.