First I'll say that the Stairway series of articles are one of the most useful set of resources I've seen on-line for SQL. I'm one of those 'Accidental DBAs' who is trying to make good. I'm actually a little surprised there isn't already a Stairway series for SQL Security, but might suggest it as a viable topic.
Possible topics (off the top of my head as a neophyte) might include:
Logins, Users, Schema, and Roles
Building Custom Roles
Using Schemas, Views, and protecting data
Certificates in SQL - creation, use, and gotchas
Backups, Restores, and Storage - (More than just backing up the database... what to store, and what not to keep together.)
Encryption - TDE, encrypting pwds/credit card numbers, etc.
And I'm sure there are books worth of other advanced topics that I don't even know to reference yet...