Teach online! You may ask why. I've already discussed this topic and provided you with 6 reasons why you should teach online. Make sure you read it first and decide if online teaching is for you or not.
Now, the very important question still stands - What should I teach online?
Bottom line: Online teaching is about helping people solve their problems.
People often talk about the concept of "doing what you love and the money will follow"... surely, this could be applied to online teaching: "teach what you love and the money will follow". However, it could be profitable for one and worthless for another. I don't think I need to give you an example here, and still - if you love eating pizza, that's fine but... you got it.
I have some questions for you:
Question #1: "What knowledge do I currently HAVE that would solve a particular problem that people have?" Generally, the bigger the problem, the higher your chances of success.
Question #2: "What knowledge do I NEED (to acquire) that would solve a particular problem that people have?", when it comes to teaching, you are not restricted to "only" what you know! You have so much resources especially online that it becomes a matter of analyzing, managing and organizing information and I'm not talking about violating copyright laws.
Question #3: "What problems have I ever had (and solved) that I would be willing to pay somebody providing they had the knowledge I needed?" The chances are, people too often have similar problems.
Hints on what you can teach (mega niches/categories): Business, Personal Development, Marketing, IT and Software, Health & Fitness, Design, Productivity, Photography, Lifestyle, Music (Play the guitar), Languages (English in particular). For more ideas, you can browse the course categories of Udemy.
Bottom line: You need to validate your idea (the problem you'd like to solve).
Once you have the answers to the questions above, go further and do a research on the topics (ideas) you've written down. You have to validate your ideas. Use www.amazon.com and have a look at the books written on your topics. What is their "Amazon Best Sellers Rank"? When you type your main keyword, say "Marketing" do you see at least 3 books in the results with an amazon best seller rank under #20000? Do these books have many reviews? When were the books written?
Go to www.udemy.com, a marketplace with more than 30k courses and 7 million students worldwide. Use the search box and type few keywords that best describe your ideas. Analyze the results. How popular are the courses displayed? How many students do they have? How many reviews do they have?
Use social media website to find out if there are discussions about your ideas going on. Facebook groups, twitter lists, subreddits in www.reddit.com. Some teachers use Google trends to determine the "interest" performance, but to be honest I am not a huge fan of it, you can still check it out though.
Generally speaking, online teachers who are beginners focus on question #1 (the knowledge they have) and go from there. However, more experienced teachers focus on question #2 (the knowledge they need, even though they do not have it right now, they will find, absorb and then teach).
Summary: Teaching online is about solving problems. You can use your existing knowledge to teach or you can acquire new knowledge for the purpose of teaching it. No matter which way you choose, you have to always validate your idea(s)!