Here's an example of something I think would be fantastic content for a marketing firm. Around a year ago, Walgreens which is a very well known drug store in the US (at least the part I live in), told all its employees to end every interaction with "Thank you and be well." They had to have done that for a variety of marketing related concerns. Discuss those concerns and what you believe their rationale for the change was.
Why is this good content? Everyone around here noticed the change, even if it was subconscious. The moment you pointed it out, people would think themselves "I did notice that actually or yeah, now that you mention it, they do do that." Then, you break down "big corporate strategy" in simple terms for everyone to understand. You come off as an expert, and if you can explain big company marketing, surely you can handle a mom and pop store. Granted, that last part isn't necessarily true, but lots of people follow that line of thinking.
Essentially, I'd just pay attention to changes in marketing that your everyday customers would notice and explain them as best you can.
Also, I think interviewing your clients and/or using them as case studies can be effective, too.
Best,
Ruben