I don't see Akainu vs. Sabo as far fetched tbh.
Oda, when saying Magma > fire, specifically pointed out that this alone cannot establish the outcome of a fight. How would he prove that statement? With Kaido, we saw how, when amplified with Haoshoku, a fire breath can become an island sized lava-like stream. With Sabo being Luffy's blood brother, portrayed close in strength with Ace and now being hyped as a figure equivalent to the Emperors, he will have strength to back that up as the literal "Emperor of the Flames". Imagine awakened Sabo using island sized blue flames he amplifies with Haoshoku, along with having great fighting skill and Haki (internal destruction Haki teased with his destroying things from the core).
From a narrative perspective it makes sense too. Sabo took Luffy's place in the Dressrosa arena, he might take Luffy's place in the fight with Akainu too, being the new personification of Ace, blaming himself for not being there back then in Marineford, telling Burgess that while he himself doesn't hold a grudge, Ace through the Mera Mera no Mi still seems to do so.
