I think YouTube math lessons is a terrific idea.
In America there are millions of home school families. Mine is one of them. If you can come up with a presentation that appeals to this market you could build a very profitable business.
I think now would be a great time to start too. It's a new year so families are getting back on track and refocussing their efforts. Many of them will get on line looking for material. Why don't you find some 1st grade thru 8th grade lesson plans to see what the US schools are doing, then every day do a lesson for each grade that is more concise, more interesting, and contains a little Greek flare to catch the attention of the US housewives (note: do not blur the lines with what Brickhead specializes in).
This would be a lot of work and very tough but if you are talented and can put on some kind of age appropriate show for each level, like a puppet show for 1st graders etc, you could get very wealthy. I just looked, StacyPlays makes 600 grand a year for that crap she posts. A quality, entertaining YouTube show teaching math could get very profitable over time. It would not be quick bucks though.
My wife was a math major and is a very good teacher. I think I might force her do this idea.