Don't think it will be Akainu because the Magma fruit is outright stated to beat the magma fruit. I see it similar to the interaction between Gomu Gomu/nika, whatever, and Enel's Fruit (Goro Goro). Fire just helps Akainu create more magma.
After the result of Ace and Akainu's clash, it would be inconsistent to have the fire fruit beat the magma fruit, when the latter is used by a character as powerful as Akainu.
Remember Ace clashed somewhat evenly with Aokiji when using fruit powers, and it was never stated that in One Piece Fire > Ice. Sometimes they're portrayed as equals, or rivals. When he clashed with Akainu, he got wrecked. The difference in power betweek Akainu and Aokiji wasn't nearly that big.
Then again, inconsistent or not, it might not stop Oda...
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