Here is an interesting thought:

How about an Arena? Think FFX (10)

Once you slay a certain monster (maybe not all monsters?) you can save that into your bestiary.

Think now of a spot in the city, where you and your friends could party up and summon the monsters that you have previously beaten. This would be a great way to hone skills, practice certain strategies, etc.

Just a random idea, found it kind of interesting.