What a thoughtful topic, here.
I’ve several monetization plan in mind, but after reading the whole topic, I think I’m leaning more towards this case:
Pay for mid-playhrough.
I planned my game to have a quite list of achievements. These achievements are mostly intended for “I’m going to 100% this” kind of players, but I think casual readers might find it interesting too.
One of the example is to play as a normal person, build your character into a vampire, and end the story as that vampire build. This way, you can get exclusive vampire-build scenes along the story and get an achievement for it.
Now, there’re also another smaller but fun achievements that related to discovering secrets, but they might be hard to find. You might miss them even after several playthroughs.
What I have in mind is having a “play this chapter” system that can be unlocked by both finishing the game at least once (showing its availability) and buying it (unlocking it). Kinda like New Game +, to an extent.
However, this system itself is not mandatory to 100% the game. It’s just helping you in the effort of 100%-ing the game by giving you the chance to explore a single-whole of the chapter to unlock its secret tidbits.
In fact, you might find it more fun by playing the whole game all over again and unlock the acievements instead. Besides, how can you know which chapter that contains that particular hidden achievement, hmm?