I had it already written in the news section:
“And finally, at last it’ll happen. The battle between that character and that character may lead to unexpected results!!”
This “battle” was already announced in last year’s Festa Letter. And no, Oda does not mean the battle between Kizaru and Luffy. He wrote in the letter from last year, that the battle between that person and that person happening would surprise us. However, the clash between the admiral and our favorite Yonko is not surprising at all, but rather was forseeable. And if Oda meant this fight, he would have used both of their names, because now that the fight has already started - it would feel awkward to not use their names in the referrence.
No, the fight Oda is meaning has not yet begun. But he will reveal the two participans soon. And the reason why I think so is: This year Oda took multiple breaks – short and long – it amounted to more than 6 or 7 weeks in total (if not more). This means that we should have seen this fight happening around October or November. However, Oda clearly is under a lot of pressure in regards of time, because he promised us a clash between `that person and that person´ since Dec 2022 in his yearly Festa Letter spoiler for 2023. Since we have not seen it happening in the manga this year, Oda does everything to at least give us that in the next few weeks. Just look at chapter 1098. He was so much under timely pressure that he could not finish it properly and published it as an unfinished manuscript. So, this means that we will get at least one more clash of two characters in the next few chapters - and we are not aware of this in the least.
Now, that we do know the outlay of the general situation, we can with almost certainty say, who those characters will be. Since Oda said that this fight will surprise us in the Festa Letter for 2023, than this means that the mangaka will heavily lay his focus on this battle. This alone means that it will have a clear impact in the overall main story line of the manga and this means: It will happen in Egghead. Here, we have two possibilities:
Kuma vs. Saturn/Kizaru: Kuma is flying to somewhere we do not know. This somewhere could be Egghead. If Bonney is in danger, than it would fit plotwise for her to be saved by her father, since Luffy is still down. This way we would have Saturn vs. Kuma. However, since we do already assume that Kuma is maybe heading towards Egghead, I do not really think that Oda meant this fight. It is not "surprising" enough to fit the criteria we are looking for.
Lucci vs. Kizaru/Saturn: Lucci is not on the WG’s side and this plays into his cards (No one seems to know it, as everyone assumes he is on the WG's side. Believe me if I say - Oda has throughout the arc, and on multiple occasions hidden hints about Lucci going against the WG, which most readers have not picked up, yet. Oda will reveal Lucci's true alliances in the next few chapters - and he is probably going to do that with a "boom" and having fun doing so. As it is, what no one expects.) This fight would, in comparison to the Kuma/Saturn,Kizaru fight, clearly be surprising to almost all reader. I would put my wager on this constellation, if I were to bet with someone.
As for the unexpected result of the battle, I have a suggestion that this is heavily tied to Ace as a character and how he died during Marineford. Because if Oda already has mentioned that the battle will change the current situation, we can safely assume that this situation, which is created after the battle, will have it’s own level of tension.
And look, currently we have the same situation like in Marineford: We have the navy, who is heavily present, we have a Luffy, who wants to flee with his friends, we have a father-figure like Whitebeard (here it is Vegapunk). The only person, who is missing is Ace as he was to be executed: In this case, Oda will use Lucci (Lucci is a twin-like brother of Shaka and therefore might be related to Vegapunk.).