Since there are 5 gears in a car and a reverse, I think Luffy will have two more gears, actually three more since newer cars have 6 gears nowadays. Anyways Gear Second lacked power, Gear Third lacked speed while Gear Fourth had both but in order for a Gear Fifth to happen Gear Fourth needs to have a weakness and I'm not talking about the drawback. I mean there needs to be something about Gear Fourth that Luffy can make up for with Gear Fifth. One other thing that is similar with all of them is the pumping, Gear Second pumps his blood, Gear Third pumps his bones and Gear Fourth pumps his muscles. Gear Fifth will have to pump something else.