Thinking about settings which would facilitate a game about recruiting, training, and combining monsters.
One of the key elements to consider in these games is the "cockfighting" element.
Forcing animals to fight other animals is... pretty grim. And if it turns out those animals are sentient, that's slavery.
Obviously you can simply ignore these thoughts. Pokemon does.
But even though you can ignore them, I can't. It's impossible for me to enjoy making a world where the player does that.