Since yesterday reminded me of the old Monstrous Compendium, can the grognards join me in remembering that book and how it could be improved?
I really liked the looseleaf approach that let you have just the right pages. We needed better binders and reinforced pages, but the idea is right. But here is a thing if you are doing that in a physical medium: it needs to be one monster per sheet, not per side of a sheet, if you are adding expansions. If Black Dragon and Green Dragon are on opposite sides of the same page, a problem arises when Brown Dragons get added. Keeping expansions separate by setting has some logic, but if you cannot even combine the binders for the base game, you have a problem.
Of course, that could lead to padding. How many monsters really deserve both sides of a page of paper? You could fill it in with things like rules for PCs of that race or advanced versions (kobold, kobold sorcerer, half-dragon kobold sorcerer…). The Monstrous Compendium tended to pad with cryptozoology and anthropology. This is useless to the gamist side of RPGs but great for the simulationist. Yes, tell me what ropers do when they are not just waiting for people. I want to hear about alternate names for otyughs. What do you know about ettin harvest festivals or orcish wedding rituals? Fluff and fluff and fluff and fluff.
I think I have all of them, still.