What, where, why, and how else do I need to invest to become a multi millionaire?

I max out my 401K each year; I max out my IRA contributions for my wife and I each year; I have setup a taxable account that has an automatic investment program where I invest in a diverse array of low cost index funds every other week; I have a multi unit building providing positive cash flow each month; I have a 529 for my son where I put money away each month. What, where, why, and how else do I need to invest to become a multi millionaire?