Would it make sense to have mid-level categories for From and To? For example, "Great Migration (African-American), Alabama to Detroit, Michigan (0, 12, 0)" would go under both "Great Migration (African-American), from Alabama" and "Great Migration (African-American), to Detroit, Michigan".
Then the latter mid-level category could be added to the more general "Detroit, Michigan" category. Then people looking at Detroit more generally would be more likely to come across all of these. Currently, they are somewhat tucked away.
The statewide mid-levels could be added to the state categories, or that state's history sub-category if applicable.
(I am not intending to add more to your to-do list! Just wondering if it would be ok to start this)