A lot of people think it will be devil fruit awakening related, I’ve seen stuff like bounce house and vulcanization. But Gear Fourth Bounce Man was in my opinion a purely Haki based upgrade of the previous gears. While Gear Fifth could completely plausibly be fruit-based, I think it’s more likely that Haki will be the main feature as that seems to be the focused power in the New World.
Full Body Armament Haki + Gear 3 or 4 size growth but compressed into the size of base-Luffy
By using Haki we have seen Luffy manipulate his body to a much greater extent in Gear Fourth. He changed Gear Third from bone to muscle balloon and then coated his relevant parts in Haki to control their movement.
This is very similar to Gear Fourth but much more mastered and Haki-exhausting. Luffy will first blow up his muscles or bones to Gear 3/4 size but then by using Full Body Armament Haki he will compress his body down to a small size. He would have full manipulation instantaneously over the size and shape of his body and retain all of the power Gear Fourth has while not releasing steam or bouncing.
I think a form like this would be super badass.
*Theory by rega619