I feel like we might get a fake-out death for Luffy. That way Oda can have his cake and eat it too. We get a "tragic" scene of Luffy sacrificing his life to save his friends and the world...and we also get him to continue his adventure forever, as he sails off into the sunset and into legend.
Luffy "dies". Then we get timeskip a few years ahead. And suddenly there's this "random" guy in a clearly fake beard running around Fuusha Village. Everyone in the village recognizes him, but just goes along with the act. Everyone pretends to not know it's Luffy, as thanks for saving the world. So, that he won't be hunted by the Marines anymore. "Mommy, why is that man wearing a fake beard?" "Shush, sweetie, he helped save the world, he can do anything he likes." But, this being Luffy, he just can't help yelling at the top of his lungs that he's the King of the Pirates. Big News Morgans is somehow right there, ready to write this story for everyone to learn about. "Strawhat Luffy...the Eternal King of the Pirates...is ALIVE??!!! This is...the BIGGEST NEWS!" Everyone can't believe how dumb he is, and the Koby along with the Marines really don't have any choice but to pretend to chase after him again. Along the way while on the run, maybe he ends up giving his straw hat to Makino's still unnamed child, now around Luffy's age when he got the Gum-Gum Fruit. Continuing the endless cycle of this story. And Luffy makes it back to the dock to hop aboard the Sunny, with his crew ready to go on another adventure together. Maybe shooting off into outer space with Franky upgrading the Sunny with rockets.
But, I don't think he's actually going to die.
...Without getting resurrected, at least. At most, we might see him get "reincarnated", if the whole reincarnation theory idea is actually true. If the prototypes Romance Dawn versions 1 and 2 really do turn out to be canon, like a lot of the stories in Oda's Wanted anthology seem to be.
Maybe one of the old Strawhats even gives younger-reincarnated-Luffy his own strawhat back. I could see Old Man Usopp telling the Strawhats' story to a group of kids, with one of them having fallen asleep in the back. Old Man Usopp recognizes the kid. And the kid tells Usopp that he sucks at telling stories. So, Old Man Usopp just laughs and tells the kid to "go out and make a better story of your own then", and gives the kid the straw hat.
So, there is a couple different ways I could see this story ending. But, I don't think we're in for a downer ending with One Piece. It's just not that kind of story.
...Unless, Oda dies before finishing it.
Because I could see whoever would end up taking over the story "honoring" Oda by keeping Luffy dead. Making Luffy a parallel for Oda himself. Maybe even having "Gear 6" make Luffy pop out of regular reality altogether and turning him into a "Cosmic Mangaka" who has to help his friends change the "bad ending" of the story by literally drawing a new ending. Luffy dying form exhaustion slumped over at his "cosmic drawing table" with a smile on his face, having finally finished his work. In that scenario, or something similar, I could see them keeping Luffy dead. But, hopefully, that doesn't happen. Both for the sake of the story, and for Oda's sake.
It really feels like Luffy should stay alive. At least to me. It could end with Luffy dying. It's not impossible. But, even if Luffy dies, I'm not so sure he'd stay dead. If a idea is memorable enough, it tends to stick around, after all. And Luffy a memorable character. I think he's going to live on. In one form or another.