Gameplay is the visible result. The work I care about is the layer that makes it reliable for players and usable for the team making it.
A team needs a mechanic. Make it usable in-editor. Reject bad data early. Ship predictable behavior.
Nerve Quest: a graph editor compiles branching quests to flat runtime data and refuses invalid connections at authoring time.
Developer Console: typed commands generate their own UMG interface, taking a balance tweak from a half-hour recompile cycle to about thirty seconds.
Dynamic Object Pool: a measured spawning bottleneck became a thread-safe generic pool that restored a stable 60 FPS.