I think they should develop a greater ecology of monsters. Make them less static. Give them more behavious (migrations, motives, personalities). For example, marmots should run around collecting food, flee from aggressive mobs, and perform courtship dances with other marmots. Beastmen tribes should wander in groups between their outposts, laying waste to any players that get in their way. Everything should have a motive and change dynamically depending on their interactions with players and the environment. Nothing should be a fixed variable.