What niche are you in or offer SEO services for? I'm trying to get an insight about other niches that SEOs are reaching out to other than plastic surgeons and lawyers.
For SEO, the sky is the limit. Could you elaborate on your question so I could think what to recommend.
You can use SEO in anything. It's not just effective for medical or legal services, of course. Here's a thought: If you want your website to rank higher in search engine results, you can use SEO.
Just be sure not to use high-frequency keywords because the competition is high for them. It's better to focus and single out 3-5 keywords and dwell on them. In this case, you'll have more chances of success.
Local businesses are easiest to get SEO results. So, instead of picking a particular niche, think about picking local businesses.