If you want to generate income with ZERO capital, all you need to do is to be extra smart and be intelligent enough to earn much in a job. Now, you have a business called "Job", where you become a physical or mental labourer and work for someone. It requires no capital but the passivity score is the lowest (1) because any day you stop working, your income also stops with it. I have not given a score of zero for job because there is a little bit of passivity in it. They are experience, industry contacts, promotions etc. As you become a senior manager, the amount of hours you work is the same but your income is higher. But still you have to keep working, WHICH SUCKS! 

Now if you have low capital and still want to make a lot of money, you can make it up with intelligence leverage. For example if you are an engineer in Google you will make more than what many small business owners will make. 

If you do not have enough intelligence, you can make it up with more capital. If you do not have enough capital you can make it up with more intelligence