This theory is more of a look into the past of the series, re-contextualizing it based on new information we’ve gotten, especially with the introduction of Rocks D. Xebec.
Should we take into consideration the obvious connection between Blackbeard and Xebec, and assuming Shanks knew Ace’s true parentage, it’s not far fetched to then say that Shanks was trying to prevent a clash between the two in order to avoid any other major calamity – not because he knew that Ace would get captured and executed.
But because he somehow knew or felt that the two “wills” of Roger and Rocks clashing once more would upset the balance of the world.
Which now puts the battle in Banaro Island in a slightly different light.
The fight will likely have parallels (could actually just be visual callbacks) with the fight in God Valley, the last time the two “wills” of these men clashed.
*Theory by yennandano