Themes
A storyline's themes usually relate to the backstories, motivations, and flaws of the player characters and villains. For example, if you've chosen revenge as one of the themes of your game, you might introduce a villain whose quest for revenge tears their life apart and causes tragic harm to those around them. You might choose a theme of love, leading to nonplayer characters involved in doomed romances, seeking to regain lovers they've lost, or courting the player characters.
You might choose to mix elements of classic high fantasy, such as the content of the Pathfinder Roleplaying Game, into your campaign. See Chapter 5: Anachronistic Adventures for advice on how to combine this material with a Starfinder campaign, or vice versa.