This is similar to the game Teleglitch. So I'll provide my suggestions based on that game:
1. Add some contrast in color and brightness between the background and your creatures.
2. Add a variety in your creatures' behaviors.
3. Make it easier for a new player to figure what special items are. Maybe make a text box will appear above them when you stand on top of them?
4.a. Consider adding objectives or side quests cause the map is big.
4.b. Add some incentive to explore them maps.
5. There is a green slime thing that blocks fire but it looks flat. Maybe change that?