I'd definitely prefer the Blackbeard pirates last....I'm not sure that's going to happen, but I'd definitely prefer it.
They're the anti-strawhat pirates named after the most famous pirate ever, two devil fruits guy with his crazy wild wacky crew...who wants to be pirate king, who's ridiculous and silly, who planned for years as a unknown guy without a bounty, philosophically similar to the main character, with TWO devil fruits, declaring war against everyone at Marineford, who has honestly done more to move the story and his plans forward than most any villain in this series. How awesome were all his scenes at Jaya. And they're pirates!
Akainu, on the other hand to me, is a bit, boring stoic cookie cutter fascist character in the series. Reminds me of the one-dimensional racist Hody, who is basically a worse more boring version of Arlong. I understand that's The Point, but what I'm trying to say is that the most realistic villains are sometimes too boring and the weird dumb assholes are just...more fun. And I hope Oda doesn't make the final boss like...Imu. Akainu at least has the Ace thing, Imu wasn't introduced until like the last four years.
I disagree that Blackbeard should be the last obstacle-- because the battle for Pirate King is only a personal beef, not one that will change anything for the World of One Piece which where systemic change, transformation of the World is what is Oda keeps underscoring as destiny, as necessity in this universe.
Furthermore, I think in story telling we're already heading on this course-- with Red Hair saying that he's going to go claim the One Piece (setting up another interesting rivalry), and mot importantly: LUFFY SAYS THAT
PIRATE KING IS NOT HIS ULTIMATE GOAL, but a PRE-REQUISITE to reach his Ultimate Dream. That Ultimate Dream that only his current crew knows about as of now, and which to them seems even crazier than the goal of becoming Pirate King.
Black Beard and Shanks are ultimately rivals in the arc that will focus on the goal of becoming Pirate King, but the step and story beyond that-- I think only the World Government and the needed systemic transformation of the world can fit that spot.
The world did not change when Roger got the One Piece. Roger realized that if he had the One Piece and more time, a "New Dawn" could come, but he would not be the one to do it. White Beard said "It's not you, Teach. You're not the one Roger is Waiting for."
So for Roger's wait to be fulfilled, Teach has to first lose in the battle that will prove him unfit. That's where we're headed at the end, and that will be the ultimate clash with the World Government.