Note: thus is just a mere theory i thought of after reading one piece.
my theory is that after luffy and his crew found all of the red poneglyphs and arrive at raftel and finally knowing what roger found out on that island, black beard woyld then appear and capture Luffy. Once he captures luffy teech would then give luffy to the marines and start the last war that could happen between the marines and pirates. Luffy would then be executed like how roger was executed on the island near the entrace of the grand line or maybe where ace is executed (either way works).
the marine would then announce the execution of the "new pirate king" luffy or "the pirate with the highest bounty" to get peoples attention and let the people know how strong the marines are.
after the execution is announced all country that luffy saved and all the pirates that are under his "fleet" would then make a plan to save luffy on the day of the execution. this could include people from alabasta, dressrosa, and all the island that the straw hats saved. it would also be the reason that garp would become the enemy of the marines. we all knkw that garp hate pieates but if its family its a different matter for him and this time sengoku might not stop him like what happend in ace's execution. maybe koby would also help luffy and maybe he could already be a vice admiral since right now hes already a rear caltain( i think).its also possible that shanks would actually make a move. its also possible that big mom or kaido would be taken out by the marines to help them fend off any other pirates that dares enter the execution grounds.( since we all know that luffy dont kill any of his enemies and almost all of them ends up in impel down).
this would be a full blown war between the pirates and the marines.
(remember what ussop's dream is before they enter the grand line? he said he wants to be known as a brave warrior of the sea and would die happily if this happens. with that said i think ussop would be the one to save luffy and remove the sea stone shackles on luffy while zoro, sanji and the rest fight the admirals and vice admirals. the one that would kill usopp would probably the gunslingeron black beards crew or the fleet admiral himself.) this could be the reason why luffy would tell the world what they found out from the poneglyphs and the tresure one piece.
we might be able to see garp, dragon, and luffy fighting back to back. luffy could be fighting black beard, garp could be fighting the fleet admiral, and dragon maybe hell fight one of the yonkou or maybe both or he could be helping garp fight the admiral with the eleders (looking at the elders one of them has some sort of sword and some has scars so maybe they can fight). the other pirates would be fighting the perfected pacifistas and the marine soldiers. zoro, sanji, and jimbei could be fighting kaidou and nami robin and the rest of the straw hat could be fighting big mom.
on the end i think luffy would do his signature of defeating enemies by punching them and destroying what they do to that country like how he defeated arlong destroying arlong Park, how he defeated doflamingo destroing the the town that mingo tried to rule. luffy would destroy mariejois punching blackbeard through it.(for me blackbeard makes sense as luffys final enemy since hes the reason his brother and whitebeard died).
after defeating blackbeard and the marines luffy would the destroy the red lune using sunny that fankie might have modified to be pluton (a weapon yhat could make a whole island disappear). luffy would destroy reverse mountai and mariejois using it.this would make the while see acsecible to everyone
after the war all their dreams would finally come true zori being the strongest swordsman defeating kaidou, nami mapping out the whole sea, robin finding all poneglyphs, frankie making sunny go around the world, chopper being the best doctor(maybe he'll cure luffy from the same disease that roger got maybe luffy will get it at raftel?). brook meeting laboon again in reverse mountain. sanji discovering all blue which is literally the whole sea that is not being separated by rhe red line and the calm belt. usopp being known as a brave warrior of the sea. and of course luffy being the pirate king connecting the whole world together and ending the reign of the celestial dragons. and maybe koby would be the new admiral replacing akainu.
edit: remember what madam sharly said on fishman island? luffy will destroy fisman island is what she said. and since fishman island is somewhere under the red line that mean if luffy destroy the red line it would also destroy fishman island and the fishman would then go on land and luffy could now fulfill his promise to shirahoshi.