Game design is mostly consequences.
Creature systems look simple from the outside, but each creature sits inside a web of rules: stats, abilities, moves, evolution requirements, progression pacing and interactions with other systems.
The interesting part is not adding more rules. It is making rules that create interesting decisions.
Data structure
Organising creature definitions, move pools and progression in consistent formats.
Rule logic
Designing evolution and ability systems with clear dependencies.
Balancing
Thinking through how one change can affect pacing and relative power elsewhere.
QA mindset
Checking edge cases and contradictions across a system with many moving parts.