When it comes to perfecting Gear 5 even further, the only solution that one can figure out is Haki. Of course, Luffy can always increase his stamina and figure out how to prolong Gear 5 ...
Monkey D. Luffy has always been criticized as One Piece's protagonist for one aspect which has now become a trait of his Gear 5 transformation.
Luffy is an exceptionally powerful figure right now ... but in action media as a whole. Even after Gear 5, many fans consider Gear Second to be iconic on a completely different level.