I mean a lot of that really depends on how the individual game itself works and even then adding some of that stuff involves adding a lot of new content.
Take car damage for example. The game as is has no way of handling that. So you have to add in health for the cars, a way to count the health down, something to handle destroying the car when the counter hits zero, the graphics for it, and the ability to do that for all of the cars in the game individually. That is not only a lot of content, but a lot of new information being processed by the game at once.
I just thought of a pretty fun, small to medium sized, and probably totally possible idea. The way the wanted level works in this game looks to be that for every level more citizens spawn as police instead, the police will always walk towards you in order to attack. Someone could come up with a way to utilize the in game clock to incrementally raise your wanted level over time, make it so killing doesn't raise it, and if there is some kind of basic collision system in place, make it so that if police touch you, you get damaged. Then make it so that every time you kill a police officer it raises your money count. Get rid of random money drops from pedestrians. Then replace the police sprite with a zombie sprite. Boom. A simple zombie survival sandbox game that can even keep score.
Possible ideas to expand on it that may or may not be possible could be like making it so if a zombie touches your car while you are driving it, it still ticks away your health so you can't just keep plowing through them in a car. That way cars would only really be used for escaping the horde. It wouldn't be terrible to decrease the speed of random cars driving by so that it would be easier to steal one either. Also maybe instead of money being dropped by pedestrians, the money pick ups could give you a small health increase instead. Honestly that seems like it would be a pretty fun little arcade style game.