Here are a few random ideas:
If you can truly master one aspect of Japanese, you could consider holding an online workshop to teach that aspect. eduFire is a service that will help you set up an online workshop and give you the ability to charge for it, though given your experience level, you probably couldn't set too high of a price. You may also find that the act of teaching others Japanese will help you learn it much faster as well.
Work in a Japanese restaurant. You didn't explicitly say you wanted to start a company ;-) If you work in an environment where speaking the language helps (e.g. speaking to customers and speaking to the restaurant employees, assuming they speak Japanese, of course), you'll bolster your learning plus make an extra buck. This applies to any shop with Japanese-speaking employees too.
If your desire to make money while learning Japanese is, in part, to help pay for the Japanese language lessons, you could barter your services with the Japanese language instructor. Perhaps you have a skill or talent that instructor needs. That would save you on lesson fees and, while not necessarily make you extra money, will lower your expenses.
Blog about you language learning experience. Post examples of difficult phrases etc. Sell language learning books and materials from the blog page.