How could core concepts be identified?
If concepts are learned based on previous concepts, then how could you automatically rank concepts based on how essential or foundational they are? In the second brain architecture, this could be done using graph-theoretic approaches. With a less structured representation, centroids could be identified. For a more low-tech approach, one could simply track word frequency usage over time to infer somewhat of a concept tree, a timeline of sequential concepts used as building blocks.