Pick something, anything, from your list of email ideas and write a shitty email about it and send it (you do keep a list of email ideas, don't you? If not, get Drafts on your phone and set up an action to append a note to some master list of email ideas you keep somewhere in the cloud, or set up some similar thing in Evernote, or use voice notes, or something. As you go about your daily life get disciplined about taking down fragments of ideas for future emails. Your future self will thank you.)
Comment on relevant news or current events in your clients industry.
Revisit an old topic/email with something new or different.
Tell a story and find a way to link it to your theme.
Solicit input from your audience. Learn more about them.
Run a crazy idea past your audience and then solicit their reaction.
Ask a question people will enjoy answering.
Point to somebody else's content. Promote somebody else's thing if a subset of your audience will find it valuable.
Point to some podcast you've been on, other content asset you've created, etc even if it's kind of old.
Link to some sort of "thank god it's friday" or "it's monday, you already need a break" kind of online humor.
Answer a common question you've gotten from clients.
Create some kind of useful checklist.
Give your take on a controversial topic within your area of focus.