[d0m]

I'd take a subject that busy engineers would be interested in knowing more about but don't have the time it take to investigate it. I.e. going in depth about how the chrome debugger work in javascript to analyze performance. I remember reading a blog post about it and a few weeks later it really helped me.

You could write about new programming languages or new algorithms that just came out. Anything with nice animation is a plus. (I.e. to illustrate the algorithm)

Another thing that I find interesting is interviewing great people in the field. Often, there are questions I wish I could ask them that most other interviewers don't because they're not technical enough.

You can write about weird bugs that you encountered, how you fixed them and what lessons you learned.