As is who fights Akainu.
Let's wait until it actually happens.
No, its not. Narrative absolutely shows you who Oda cares about and their relationships. Oda has had Luffy witness Ace's death by Akainu. Oda had Luffy scarred by Akainu. Oda had Luffy traumatized by the event. Oda had Luffy's scar throb when Akainu was mentioned post timeskip. Sabo literally has nothing to do with Akainu whatsoever
Headcanon is when you provide an example that is a complete fallacy to the argument. There is no scenario where Sabo's emotional payoff for Ace is higher than Luffy's. There is no resonance there to even suggest he is "taking over" that fight. Everything you mentioned applies to Luffy multiple times over, because he's the MC of the story
I even provided you a myriad of examples where the Revolutionaries goals involve the Celestial dragons, and are preparing to face the God Knights, not the Marines.
But sure, its "headcanon" to think Luffy is facing Akainu