"And finally, at last it'll happen. The Battle between that character and that character may lead to unexpected results...." - Oda
The best bet has to be Shanks vs Blackbeard. Is the only long running pending battle that can lead to unexpected results with BB victory.
Shanks vs Mihawk, doesn't fit too much considering Oda's words. If it was about them, he should have said "finally, at last it will happen again" which doesn't make sense since both lost interest picking battles between them
Didn't Shanks and Blackbeard fight before? Is that why Shanks has his scar?
In my understanding of the phrase, "And finally, at last it'll happen," Oda will "finally" resolve the debate between the two by having them fight again, which will be their "final showdown." When Oda says, "it may lead to unexpected results," he might be hinting that the "future" of the Cross Guild will be something fans won’t expect, based on the aftermath of Shanks and Mihawk's fight.
I don’t think a Red-Haired Pirates vs. Blackbeard Pirates battle will happen soon. You mentioned that Blackbeard would win if they fought now, but I disagree. There are still many things we don’t know about Shanks compared to Blackbeard, and Shanks is a more important character. Additionally, Blackbeard had difficulties fighting Law, so suggesting he could defeat Shanks—especially after Shanks easily defeated Eustass Kid—seems unlikely. Blackbeard would need a significant upgrade in strength and power to fight Shanks on equal terms. And if you think Blackbeard would cheat in their fight, I don’t believe that will happen. Shanks has fought Blackbeard before and is already aware of his strategies and schemes. If Blackbeard were to cheat, it would likely be in a fight with Luffy or Buggy, where they wouldn’t yet know his real power.