The idea that Palpatine’s goal was to prepare for the Vong. If that was the case, what was the goal of the Sith Lords before him, and why didn’t he warn anybody?
Well, that’s debatable. From what I’ve gathered it’s more of an in-universe justification of Palpatne’s actions post Yuuzhan Vong invasion by members of the Imperial remnant. Conquering the galaxy in order to protect it, and not warning anyone, is a pretty weird plan. I’d say his motivations was still as a Sith, but since Legends is a pretty extensive expanded universe, it makes sense that he was at least somewhat aware of the Vong, as well as preparing for it in some way.
Eckhartsladder made a video about it, arguing that Palpatine’s motivations were primarily based on the Sith doctrine and his own desire for power; https://www.youtube.com/watch?v=vTGwVVa4B2U