Complex Game System Design and Implementation Questions

Design and implement non-trivial game systems from requirements. Systems might include: inventory management (add/remove items, weight/capacity constraints, sorting), save/load systems (serialization, data validation, versioning), progression systems (experience, leveling, unlocks), economy systems (currency, pricing, transactions), or game mechanics (health/damage, status effects, buffs/debuffs). Your solution should handle edge cases, be thread-safe where applicable, and account for data persistence or networking.