Akainu is surely not getting Blueno'd, that much is sure but are we that sure Oda is not going to go the Sabo route?
Sabo inherited the Mera mera and thus Ace's will,it is Oda's style to atleast involve Sabo in Akainu's defeat.
So I think it will be Luffy and Sabo vs Akainu at the start of the fight,then Luffy goes on to more important stuff and Sabo finishes the job.
What do you think?
I don't think that there any chances of someone being in the place of Luffy when it comes to his fights with BB and Akainu.
For what regards Akainu and Luffy we have :
1)Enormous personal history.
2) Akainu also gave him his only scar (and scars have great importance and underline important relationships in One Piece, see Oden and Kaido, Zoro and Mihawk, Shanks and BB).
3)Akainu is the guy who almost broke Luffy, the guy who is the ultimate and sworn enemy of piracy, the guy who wanted to care of Luffy and every cost.
Akainu will get in Luffy's path.
In the new world, when the important changes in the world are being reported by Jinbe, Luffy stays calm at BB but his scar aches and his expression drastically changes when Jinbei mentions Akainu. We see that he holds a big grudge.
It's also no wonder that both Akainu and BB have paralleled Luffy in some way : those three all grew during the timeskip. BB became an emperor, Akainu became the fleet admiral.
There are also other two panels at Marineford where both BB and Akainu respective and flawed ideologies are put side by side. I don't think this was unintentional by Oda as well.
4)Big, big contrast between Akainu absolute justice and Luffy desire for freedom.
5)A great marine that fights a great pirate. This explains itself.
6)In Dressrosa, Luffy also says that he needs to defeat every admiral and emperor. And I wonder who he might fight from the marine side.
To finish, I think that Luffy having a 1 vs 1 against Akainu is pretty much set in stone, just like BB vs Luffy or Mihawk vs Zoro.
I don't see any actual buildup for Sabo being the one to take him out. In fact, I personally think that it's one of the worst popular theories out there.