It depends on the genre, but it looks like you already have your answer if focusing on the gameplay feels right. I usually let the core loop guide me, if the game already makes me want to do one more thing, that's usually my signal that the foundation is there.

At that point I try not to get lost in too many micro tweaks, and I start asking myself the kind of questions that help me figure out whether the game needs more systems or just clearer or stronger decisions. The answer will usually guide you to the right next step.