I believe that the last oponent will be World Government and particulary - Gorousei as they are the head body that rules WG and hence all the World and Luffy is destined to fight the World after finding One Piece on Raftel.
Of course I might be wrong, but it's the way I see all the plot as it is told now.
If Buggy becomes Pirate King it would be the biggest troll in the history of all literature. The hero dying so that a tertiary supporting character can achieve greatness is the most patently ridiculous sentiment I have ever seen.
More importantly, fighting the World Government after Raftel would be far more dramatic. The World Government going to war against the Pirate King would be far more dramatic than the World Government going to war against the prospective Pirate King. Imagine Luffy being proclaimed Pirate King, greatest hero of his age, reveals the true history to the public, followed by the World Government mobilizing for one last stand as Luffy gathers friends and allies from Albasta to Revolutionary Army, mobilizes the ancient weapons, defeats the World Government, and the manga ending with Luffy restoring the glory of the ancient kingdom, opening a new golden age. Defeating The World Government before Blackbeard eliminates all of that. Defeating the World Government before Raftel eliminates much of the drama too. Rather than confront them in all his glory as Pirate King, Luffy would just be another Pirate who became a nuisance, and needed to be snuffed out before things got any further.
I always believe the last enemy before reaching the One Piece will be Blackbeard. Maybe on Raftel!!!
But the last enemy of the Series will be the Gorousei and the WG.
I could imagine the fight between Luffy and Blackbeard taking place on Raftel itself. That would be a really epic location for the fight! I expect that the fight with the Blackbeard Pirates will take place across multiple islands, as both crews move toward the end of the New World. I also hope that Shanks and Luffy's reunion does not take place until the beginning of the Blackbeard saga. That would be a great moment for Shanks to acknowledge all Luffy has accomplished, lay his own ambitions aside, and aid Luffy in the last stage of his quest.
With Blackbeard out of the way, I expect Robin to find the Rio Poneglyph, and the full extent of the Tenryuubito and World Government's crimes to be exposed. The Strawhats coming to grips with the True History would set the stage perfectly for the last War, so much more would be on the line than if the government were destroyed before Raftel.
Hmm i don't quite like the thought of BB actually reaching Raftel
I mean it would be such a waste . I personally want to see only the SH's finding that place which was discovered only by Roger . That BB scum doesn't deserve the privilege of stepping on that island
The final battle between BB and Luffy will take place near that island but BB will never discover it
Blackbeard and Akainu battles will happen before Raftel, but a Luffy v. Coby fight will be the last. It will be awesome and epic