The best things to do content on are the things you are actually working with and thinking about day-to-day. (This is related to Gary Vaynerchuk's idea of document vs. create.) Someone recently asked me, “Why don’t you do some content on things that would be beginner topics for Magento developers?” I think that would be really useful content. Max Pronko puts out some great content related to that. But I’m not using Magento day-to-day anymore. I could try, but it would take a lot of work and wouldn’t be the best quality.
When Phillip Jackson and I started Magetalk four years ago, we were both doing Magento development. It was trivial to start the podcast and talk about what we were doing each day. It was useful and it made sense.
Now that we aren’t doing that work anymore, the podcast has shifted. But that’s a big part of why Magetalk was successful.
So when you are putting out content, talk about what you are doing day-to-day.
The questions you are getting in your work are prime topics for content. Just shoot a video regurgitating the same advice you give clients. Eventually, you'll have a library of useful content.