One idea that could be interesting is different types of creatures and enemies for different types of worlds or areas. Maybe you could find nymphs in a dense wooded area, they could be peaceful or hostile. Yetis could possibly appear in snowy regions. A friendly water beast, such as Nessie, that would rarely appear would be a nice addition to large lakes. Another thing that would maybe be cool in snowy regions is something like the tauntauns in Star Wars.
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