Gear 5 isn't the final power-up for Luffy. Gear 6 is still a possibility in One Piece.
Luffy could have another transformation after Gear 5. Oda has already set it up in One Piece.
The reason behind that is quite simple. The first thing that fans need to consider is that Luffy will be looking to master his Haki in this particular arc of One Piece. Luffy's Conqueror's Haki is ...
Luffy's journey has brought him across some of the most important characters in the world, but he hasn't met everyone.