This is a great game, and I really liked it! I would love to see this concept expanded if you can. If you do end up expanding this, I have a few ideas. I think something cool to incorporate would be to have Harper and Ollie meet more people and also have some of them start writing to you. We could then get a bunch of different perspectives that could allow for cool puzzles.
I also think it would be cool if some of your friends occasionally wrote to you asking for info on monsters. You would have to use the letters to put together the relevant information and then write them back with answers. I think this would make it feel more like a back-and-forth conversation instead of people just talking at you. It could also be really fun to actually use the information you learn for something.
It might also be fun to have a map you have to fill in. I liked how you had to fill in the alignment chart and thought that was cool. I feel like a similar thing could be done with the world map. You have to deduce the different regions of the world and note down their locations.
All in all, I think some more monsters, perhaps a few new alignments and habitats, and some more moves would really flesh out the game and allow it to become even better than it already is!