Yeah nobody knows what will happen
We just know that at some point Luffy will fight BB and will fight Imu
But there is no certainties at all that Luffy will have a full fight with Akainu
While it’s almost certain he will fight the 2 other
Akainu will have a big role, but having a big role isn’t equal to fighting Luffy and being a full length fight
We'll see
It's a matter of point of view
For me it's almost certain.
Because of Akainu being the only character to scar Luffy in the entire series, a throbbing scar at that. Then you add killing his brother and temporaly breaking his will. A bit too much for a secondary villain to Luffy's story.
It wasn't random that the end of fishman island, the two enemies Jinbe wanted Luffy to notice was Blackbeard rising to emperor and Akainu to fleet admiral.
In the bigger story of the OP world Akainu isn't an important/relevant as Imu or potentially Teach, but as far as personal opponents he has the most impact on Luffy.
And in the end this is Luffy's adventure.