To be as big as Michael, you would have to be known by 99.9% of the world's population and that's never happened before him and it hasn't happened since. The closest person to Michael in terms of recongntion is Muhammad Ali. No other individual person is as well known or ever has been. So, no, I don't think so and I definitly don't think that there can ever be anyone bigger, because you would have to have 100% of the world population know who you are for that to happen, and that's technically impossible. It's also less likely anyone will reach that status of fame when you consider that since Michael and Madonna, there hasn't been any legitimate superstars. The atmosphere today just simply isn't condusive of it.
Michael has a unique blend of unparalleled talent, an enigmatic personality, and pure genius. That type of blend so rarely occures, people don't understand how special it is when it does.