Here are some ideas I think could help enemy spawns:

1. Different spawns during the day and night. Some mobs could be active only during the day, only during the night, or during both. Example: bats and fungii are night mobs only. Maybe levels and powers/tactics could change as well.

2. Different spawns/powers during different weather conditions. In girdania it rains a bit... so there should be some mobs that only come out during the rain and others that go into hiding when it rains. Likewise maybe gas/spore attacks aren't used as much by mobs in those conditions, but water ones are used more often.

3. Spawn seasons. I suggest letting spawns have different seasons and how often they are seen and how strong they are depends on which season in the game it is. If it's marmot mating season, you should see a ton of marmots out and about. However if it's the warm/dry season, no fungii should be seen.