Almost at my first bingo! (Unfortunately I have no idea how to theorise about world, character OR theme— so— don’t worryyyy, I’ve totally got this)
Woohoo!! You do, you got this 👏🏻
Some tasks are def more daunting than others (bargaining firstborn for more is another that we helped people troubleshoot in the past, since it's the weirdest one).
For the theorizing square, I'd say speculating about character is the easiest angle—sort of making inferences about a character's motivations or feelings, digging into stuff that's been left up to interpretation a bit. Or predicting what the character will do in xyz potential scenario. Lots to play around with!
Theorizing about the world is easier to do with fics that feature extensive worldbuilding of the author's own making, where you can wonder about the broader implications of established lore and whatnot.
Exploring themes is probably the toughest, as it's more about major ideas and throughlines that you've noticed over the course of the text, and what underlying concepts the story examines or sheds light on.
No matter what though, just have fun with it! It's so cool when readers engage with the story on a deeper level—whatever you come up with will be great 💛