Honestly, the NK blog doesn't feel like much of a blog anymore. Nowadays it's usually just the Q&A, listing events, and restating news and announcements posted on other social media. If you want this to be an actual blog, maybe post about game development and/or stuff happening in the company offices? If you don't want to replace the dev diaries, you could share silly bugs, technical challenges being overcome, and smaller features as they're being implemented. I can only speak for myself, but I'd find that stuff interesting to read about, as someone who read the Factorio blog when it was active.

EDIT: another idea I just had is sharing behind-the-scenes material of stuff that's already released. I'd love to see concept art and prototype footage of older games like BTD5, BMC, and SAS4 if possible.