I would approve of this, something along the lines of what Ffxiii has (if there is one thing about that game I had to appreciate, its the bestiary), imagine beating a raid boss and getting new info about it and some art/model. I would treat it almost like a trophy.

On top of that, there could be achievements for completing certain open world zones. The only issue I see though are the mobs from dungeons that are used once and just have random names, which would clog up the bestiary quite a bit