How do you feel about hierarchical tags? No, I don't mean in the way that Anki has a "file tree/directory" kind of structure for tags and decks, but like, a separate "concept" hierarchical graph that organizes the tags.
For example, consider two tags, "Fractions" and "Prime Numbers". One should know Fractions before they study Prime Factors, and one could represent that using a drag/drop UI like <insert generic mindmap tool>. This "concept hierarchy" would organize using tags. This way you could still syntactically tag cards with, say, "Prealgebra/Fractions" and "Prealgebra/Prime Numbers". In this way one could have a "syntax" tree and a "semantic" tree/graph. (File trees can't represent graphs, as you may know.)
One problem with making cards directly hierarchical with each other is that people have many cards, and organizing them individually can be a pain (and questionable usage of time).
For example, consider two tags, "Fractions" and "Prime Numbers". One should know Fractions before they study Prime Factors, and one could represent that using a drag/drop UI like <insert generic mindmap tool>. This "concept hierarchy" would organize using tags. This way you could still syntactically tag cards with, say, "Prealgebra/Fractions" and "Prealgebra/Prime Numbers". In this way one could have a "syntax" tree and a "semantic" tree/graph. (File trees can't represent graphs, as you may know.)
One problem with making cards directly hierarchical with each other is that people have many cards, and organizing them individually can be a pain (and questionable usage of time).