This year I am applying to 32 schools. Although I have not received any of the secondaries, I drew the information fro last year, and analyzed their prompts, and came up this category system.
Cluster 1: Diversity (N = 13)
Cluster 2: Most Important Non-Academic Experiences (N = 2) /Work (N = 9) /Accomplishment (N = 3)
Cluster 2.5: Preparation to Medical Career (N = 4)
Cluster 3: Volunteer (N = 4)
Cluster 4: GAP years (N = 11) /2018-19 Activities (N = 9)
Cluster 4.5: Research (N = 7) / Global health (N = 2) / Most Important Honor (N = 2)
Cluster 5: Most difficult challenges (N = 8)
Cluster 6: Leaderships(Conflict-resolution) (N = 7)
Cluster 7: Hardship (N = 12)
Cluster 8: Why Here (N = 13) /Location (N = 4)
Cluster 9: Future (N = 5)
Cluster 10: Fun (N = 2)
Cluster 11: Others (N = 4)
Cluster 12: Crazy (N = 2)
Cluster 13: Anything Else (N = 11)
Disclaimer: This category system reflects my writing strategy, for example, I will write those topics under cluster 2 and 2.5 together as they may fit my overall narrative. The numbers in the brackets are how many schools had those prompts last cycle. So you can see, indeed, the diversity, adversity (most difficult challenge), and gap year questions are asked the most frequently. Yet, some other questions are asked quite a few times too, such as hardships (e.g. explaining the bad grades) and "why here/why this location". By the way, "Others" includes those prompts I don't know where to put, "Crazy" has prompts.... you will know when you see them, and "Anything else" is the prompt "do you have anything else to inform adcom?"
I highly recommend to make your own coding system/category so that you can have an overall idea, given the schools you are applying, about what topics you have to write and which topics gain high priority, and plan your writing strategies accordingly.
Good luck.