My ideas are these:
1. Going into related niches: I have this weight loss site that's making me money so it could be a good idea to start a muscle building site. Some of my visitors are men who could be interested in muscle building so I could send some of my current visitors to my new site. Then I would keep on building sites targeting men (dating/seduction, male sex issues, etc.)
2. Translating my site into English: this weight loss site is in Spanish so translating it into English could be a good idea. It can be cheaper than starting a site from scratch. Since content would be translated keyword research could be skipped. Translating can be cheaper than writing, and the site is already being profitable.
3. Going into unrelated niches that I have knowledge on: since I am a teacher I could go into education related sites. I was thinking something related to teaching English, learning English, studying Spanish, among others. This would require more work than option 2, but probably less than option 1. The problem is getting the new visitors.
There could be more ideas but these are the three I have right now. Which one is your favorite? What would you do in my case? Any suggestions are welcome