Since worlds will expand if you invest your collected pages into it, I would love to see some side incentive to do so. Like, lets say you've done the bare minimum to move on from one book/world to the next, but you wanted to find out what happened to a character(s), TO ADVANCE THE STORY STILL GOING ON WITHIN THAT WROLD!

I would love to feel intrinsically motivated to do so based on the curiosity I've gained unearthing the clues to its mystery by just following the main missions.
I would LOVE to see at least one would do this in a serious way, internal politics and drama to some degree.(By serious I mean something with human depth). This could be some I want to become attached to one world's story and characters because of the complexity of their relationship to their world.
--Alternatively, instead of a whole world...--
We could have the development of any one side character expand as that book/world does due to an investment of pagies by the player. Unraveling curiosities the player might have about the background of a particular character, or the mysteries of a certain world, or the Shakespearean complex in-depth hidden stories between characters that aren't necessarily pointed out or explained to you but hinted with small clues(for the lore hungry).
These ideas I had in mind were more along the lines of something simple:(@)an exploer who is climbing a mountain constantly or the depths of a jungle world and you get to see what he/she encounters as they progress/you add more pagies to the world.
(@)Same thing but for a miner who when you first help solve an uninteresting problem leads to the discovery of something small and interesting(like a beautiful artifact that resembles the hint of a power up for you); as the pagies expand the world, what they are uncovering becomes more serious and you can see them excited and obsessing over it(you would see the uncovery progress too, designed to cleverly leave you with more questions each time you visit); completely seeing a shift in their character(like from clean and proper to slowly more dirty and grungy everytime there's progress). I want to hear myself say "what is it?! I need to get more pagies for this!". Maybe in the end you fight them as a side/tough bonus boss (because maybe they uncovered power in the end like magic or ancient tech) for whatever the treasure is and the whole experience evolves them as a character you maybe didn't like before but feel like they've changed for the better. Like maybe as a result of loosing the fight to you they seem to then become a pacifist hippie renouncing all material possessions "dude". Light hearted humor and such
(@)Clues to who a mysterious superhero really is
What I was mentioning I would really like to see though is along the lines of something more complex:(@)Lovers in secret (you can find/see a hidden locket or photo or letter tucked under inconspicuous clutter, or by bashing an object with a specific move, or uncover a hidden room behind a bookshelf by doing something unique that you wouldn't do anywhere else in the game,e.g., like sit in a chair or something/ something out of context to the games normal mechanics)
(@)(World supporting a feud? Political, Tribal, Family.) The true guilty party of a murder that got away with it (your skills allow you to find the easter eggs, e.g., hidden room, or see behind a wall with sonar). The murder would have already taken place once you've first arrived of course; so that you don't have a personal connection with them once their wacked(although you could still develop a relationship with them if you get to know the depths of who they were) This wouldn't be an official mission, just in game lore that has detail because it was fun to develop and stick into the game. THE PU COMMUNITY WOULD GO BAT#$%* MAD LOOKING FOR THE CLUES IF CONVOLUTED ENOUGH

,.i.e., Five Nights at Freddie's convoluted lore style(People who didn't even like the genre of scary games invested in unraveling the untold story and looking for the possible connection between things

) This would be fun even if it was cartoony.
An example to try and give you an idea of what I am talking about for the more serious stuff is the type of character development you would see in Psychonauts while inside someone's mind(the level); you would explore the world and slowly unravel maybe some dark things, or maybe some interesting things that gave those characters depth and a peak into their vulnerabilities & past, overall things that weren't said but that you felt you uncovered and saw there because these characters were given personalities and you saw the dynamics of their story; you were rewarded for paying attention while playing and not just collecting/"platforming". Just under 2-min vid of a
perfect example:
TL;DR I want to see something like this (https://youtu.be/m6TXYP6qU8I) be fed to you when expanding at least one world's theme again and again. Consider building a world that supports this evolution. 
Non-related to world theme:
That incentive for uncovering what "the miner" has started to unearth would be the style I'd like to see for the player to 100% the game. Where you get to see the progress and the scene evolve until you finish dropping off all the rare special item(s) from each world or you 100% the game because you collected all of the things "needed"/ the common items. This incentive would be in the world hub most likely.
(An example would be the type of collecting you did for Snide in DK64 with the blueprints but it would be a side boss/power up as a reward not an advantage to progressing in the storyline as that was in that game)