I'll awnser that question! Because I can't imagine luffy transforming into a complete beast! And for those who read the manga we got a very cool form on Gear 4 instead of "Bounce Man" we got the "baloon...". I don't see Oda jumping off to all these gears, maybe if there's another timeskip maybe Luffy will get new Gears, but not now. He will fight Kaido with his Gear 4, and I think he will loose and get stronger but not enough to a fifth gear.
What do you think guys?